I'm using a Server 2012r2 as a PPTP Server, and it been working great until a few hours ago, when I no longer can access the PPTP server from my iPhone (ios 9.3.2). Ran a traffic capture on port 1723 and it gets connected and everything looks fine, but the iPhone says Connection could not be established. See traffic capture below. I have restarted the RRAS, the DC, and every machine in the env. Restarted the router. The port is open, I ran an nmap from an external network.
Event log:
A connection between the VPN server and the VPN client xxx.xxx.xxx.xxx has been established, but the VPN connection cannot be completed. The most common cause for this is that a firewall or router between the VPN server and the VPN client is not configured to allow Generic Routing Encapsulation (GRE) packets (protocol 47).
C:\Windows\system32>nmap -sO 10.10.10.40
Starting Nmap 6.47 ( http://nmap.org ) at 2016-05-21 18:19 W. Europe Daylight Time
Warning: 10.10.10.40 giving up on port because retransmission cap hit (10).
Nmap scan report for 10.10.10.40
Host is up (0.0018s latency).
Not shown: 246 closed protocols
PROTOCOL STATE SERVICE
1 open icmp
2 open|filtered igmp
4 open|filtered ip
6 open tcp
17 open udp
41 open|filtered ipv6
47 open|filtered gre
50 open|filtered esp
51 open|filtered ah
113 open|filtered pgm
l2tp works fine.. I guess its using GRE to.