Hello once again SpiceWorks!
Some background information: I am in the process of trying to deploy a L2TP over IPSec VPN server with our Windows 2012 R2 server. The server is completely configured and working. Non-Windows clients are able to connect with no further configuring. Windows clients, however, were quite a pain to get working. I used the AssumeUDPEncapsulationContextOnSendRule registry fix and that seemed to work. Then I tried connecting from a remote location and it stopped working again.
Here's what seems to be going on: the server is being hosted on a Verizon FiOS line. If the remote location I am trying to connect in from also has FiOS, then there are no issues for Windows clients. Whenever I try to connect from a non-FiOS remote network, the dialing hangs and then returns the lovely error 789 again. If I fully expose the server with DMZ on the FiOS firewall, then all connections work as expected (including non-FiOS remote networks). I do not want to do this for obvious reasons. I'm suspecting a NAT issue somewhere, but not sure where to start looking. I've been fighting with this thing for about 2 days now with no luck...
Has anyone else experienced this issue? And if so, did anyone find a way to fix this? (We also do have PTPP configured, but there are some remote locations we connect from that block the PTPP ports and we are looking for a more secure connection in).