NPS: Event 6274 - Network Policy Server discarded the request for a user

Intermittently I will get desktop (wired) and laptop (wireless) computers experiencing issues with NPS (they drop off the network).

Some computers are affected more than others, although they are identical hardware and based on a standard image.

In the event log of the NPS servers I can see the following messages:

Log Name:      Security
Source:        Microsoft-Windows-Security-Auditing
Date:          2/05/2014 8:47:58 a.m.
Event ID:      6274
Task Category: Network Policy Server
Level:         Information
Keywords:      Audit Failure
User:          N/A
Computer:      NT147.domain.local
Description:
Network Policy Server discarded the request for a user.
Contact the Network Policy Server administrator for more information.
User:
 Security ID:   NULL SID
 Account Name:   host/DPC0387.domain.local
 Account Domain:   DOMAIN
 Fully Qualified Account Name: DOMAIN\DPC0387$
Client Machine:
 Security ID:   NULL SID
 Account Name:   -
 Fully Qualified Account Name: -
 OS-Version:   -
 Called Station Identifier:  3c-xx-xx-xx-xx-xx
 Calling Station Identifier:  00-xx-xx-xx-xx-xx
NAS:
 NAS IPv4 Address:  10.nnn.nnn.nnn
 NAS IPv6 Address:  -
 NAS Identifier:   ND246
 NAS Port-Type:   Ethernet
 NAS Port:   71
RADIUS Client:
 Client Friendly Name:  Network Device Management Subnet
 Client IP Address:   10.nnn.nnn.nnn
Authentication Details:
 Connection Request Policy Name: NAP 802.1X (Wired)
 Network Policy Name:  -
 Authentication Provider:  Windows
 Authentication Server:  NT147.domain.local
 Authentication Type:  -
 EAP Type:   -
 Account Session Identifier:  384F322E317838316564303034313030306230666632
 Reason Code:   1
 Reason:    An internal error occurred. Check the system event log for additional information.

How do I debug when an internal error occurs but there is nothing in the system event log? Where else can I look?

Here's the packet trace that matches the event log entry above:

No.     Time        Source                Destination           Protocol Length Time from request Info
      1 0.000000    JuniperN_xx:xx:xx     Universa_xx:xx:xx     EAP      60                       Request, Identity
      2 2.470423    Universa_xx:xx:xx     Nearest               EAPOL    60                       Start
      3 2.472870    JuniperN_xx:xx:xx     Universa_xx:xx:xx     EAP      60                       Request, Identity
      4 2.539416    Universa_xx:xx:xx     Nearest               EAP      60                       Response, Identity
      5 2.544206    Universa_xx:xx:xx     Nearest               EAPOL    60                       Start
      6 2.548804    JuniperN_xx:xx:xx     Universa_xx:xx:xx     EAP      60                       Request, Identity
      7 2.550050    Universa_xx:xx:xx     Nearest               EAP      60                       Response, Identity
      8 2.552597    10.switch             10.NPS_Server         RADIUS   254                      Access-Request(1) (id=249, l=208)
      9 2.556043    10.NPS_Server         10.switch             RADIUS   136    0.003446000       Access-Challenge(11) (id=249, l=90)
     10 2.565876    JuniperN_xx:xx:xx     Universa_xx:xx:xx     EAP      60                       Request, Protected EAP (EAP-PEAP)
     11 2.569472    10.switch             10.NPS_Server         RADIUS   254                      Access-Request(1) (id=250, l=208)
     12 2.572566    10.NPS_Server         10.switch             RADIUS   136    0.003094000       Access-Challenge(11) (id=250, l=90)
     13 2.580254    Universa_xx:xx:xx     Nearest               TLSv1    123                      Client Hello
     14 2.586544    10.switch             10.NPS_Server         RADIUS   361                      Access-Request(1) (id=251, l=315)
     15 4.564841    Universa_xx:xx:xx     Nearest               EAPOL    60                       Start
     16 4.568530    JuniperN_xx:xx:xx     Universa_xx:xx:xx     EAP      60                       Request, Identity
     17 4.569876    Universa_xx:xx:xx     Nearest               EAP      60                       Response, Identity
     18 4.582263    10.switch             10.NPS_Server         RADIUS   254                      Access-Request(1) (id=252, l=208)
     19 4.586006    10.NPS_Server         10.switch             RADIUS   136    0.003743000       Access-Challenge(11) (id=252, l=90)
     20 4.591896    JuniperN_xx:xx:xx     Universa_xx:xx:xx     EAP      60                       Request, Protected EAP (EAP-PEAP)
     21 4.592692    Universa_xx:xx:xx     Nearest               TLSv1    123                      Client Hello
     22 4.599634    10.switch             10.NPS_Server         RADIUS   361                      Access-Request(1) (id=253, l=315)
     23 4.600887    10.NPS_Server         10.switch             IPv4     1518                     Fragmented IP protocol (proto=UDP 17, off=0, ID=07db)
     24 4.609920    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    1514                     Server Hello, Certificate, Certificate Request, Server Hello Done
     25 4.610516    Universa_xx:xx:xx     Nearest               EAP      60                       Response, Protected EAP (EAP-PEAP)
     26 4.617407    10.switch             10.NPS_Server         RADIUS   262                      Access-Request(1) (id=254, l=216)
     27 4.618352    10.NPS_Server         10.switch             RADIUS   288    0.000945000       Access-Challenge(11) (id=254, l=242)
     28 4.623650    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    176                      Server Hello, Certificate, Certificate Request, Server Hello Done
     29 4.643316    Universa_xx:xx:xx     Nearest               TLSv1    361                      Certificate, Client Key Exchange, Change Cipher Spec, Encrypted Handshake Message
     30 4.649607    10.switch             10.NPS_Server         RADIUS   601                      Access-Request(1) (id=255, l=555)
     31 4.656950    10.NPS_Server         10.switch             RADIUS   199    0.007343000       Access-Challenge(11) (id=255, l=153)
     32 4.662734    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    87                       Change Cipher Spec, Encrypted Handshake Message
     33 4.681106    Universa_xx:xx:xx     Nearest               EAP      60                       Response, Protected EAP (EAP-PEAP)
     34 4.788536    10.switch             10.NPS_Server         RADIUS   262                      Access-Request(1) (id=2, l=216)
     35 4.789735    10.NPS_Server         10.switch             RADIUS   173    0.001199000       Access-Challenge(11) (id=2, l=127)
     36 4.795723    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    61                       Application Data
     37 4.796372    Universa_xx:xx:xx     Nearest               TLSv1    93                       Application Data
     38 4.802368    10.switch             10.NPS_Server         RADIUS   331                      Access-Request(1) (id=3, l=285)
     39 4.803363    10.NPS_Server         10.switch             RADIUS   189    0.000995000       Access-Challenge(11) (id=3, l=143)
     40 4.808905    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    77                       Application Data
     41 4.809501    Universa_xx:xx:xx     Nearest               TLSv1    77                       Application Data
     42 4.817342    10.switch             10.NPS_Server         RADIUS   315                      Access-Request(1) (id=4, l=269)
     43 4.822986    10.NPS_Server         10.switch             RADIUS   189    0.005644000       Access-Challenge(11) (id=4, l=143)
     44 4.828973    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    77                       Application Data
     45 4.833318    Universa_xx:xx:xx     Nearest               TLSv1    829                      Application Data
     46 4.840610    10.switch             10.NPS_Server         RADIUS   1073                     Access-Request(1) (id=5, l=1027)
     47 4.845946    10.NPS_Server         10.switch             RADIUS   189    0.005336000       Access-Challenge(11) (id=5, l=143)
     48 4.850938    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    77                       Application Data
     49 4.907924    Universa_xx:xx:xx     Nearest               TLSv1    141                      Application Data
     50 4.913390    10.switch             10.NPS_Server         RADIUS   379                      Access-Request(1) (id=6, l=333)
     51 4.917535    10.NPS_Server         10.switch             RADIUS   221    0.004145000       Access-Challenge(11) (id=6, l=175)
     52 4.922877    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    109                      Application Data
     53 4.923472    Universa_xx:xx:xx     Nearest               TLSv1    61                       Application Data
     54 4.930319    10.switch             10.NPS_Server         RADIUS   299                      Access-Request(1) (id=7, l=253)
     55 4.937348    10.NPS_Server         10.switch             RADIUS   381    0.007029000       Access-Challenge(11) (id=7, l=335)
     56 4.942543    JuniperN_xx:xx:xx     Universa_xx:xx:xx     TLSv1    269                      Application Data
     57 4.944791    Universa_xx:xx:xx     Nearest               TLSv1    125                      Application Data
     58 4.951408    10.switch             10.NPS_Server         RADIUS   363                      Access-Request(1) (id=8, l=317)
     59 4.954022    10.NPS_Server         10.switch             RADIUS   355    0.002614000       Access-Accept(2) (id=8, l=309)
     60 4.981482    JuniperN_xx:xx:xx     Universa_xx:xx:xx     EAP      60                       Success
     61 32.590347   10.switch             10.NPS_Server         RADIUS   361                      Access-Request(1) (id=251, l=315)
     62 62.592420   10.switch             10.NPS_Server         RADIUS   361                      Access-Request(1) (id=251, l=315)
     63 92.595043   10.switch             10.NPS_Backup_Server  RADIUS   361                      Access-Request(1) (id=9, l=315)
     64 122.597856  10.switch             10.NPS_Backup_Server  RADIUS   361                      Access-Request(1) (id=9, l=315)
     65 152.600618  10.switch             10.NPS_Backup_Server  RADIUS   361                      Access-Request(1) (id=9, l=315)