How to diagnose L2TP VPN Error 789?

Mitan

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I need to login to a remote server using a VPN using L2TP with a pre-shared key, but my Windows 8.1 laptop keeps failing with Error 789.

I've got it running with no problems from my Windows 8.1 PC (and from my Android phone) through my home router, so the router isn't the problem.
I can ping the server with no problem.
The necessary services are auto-starting OK.
Both my PC and my laptop have identical ESET firewall configurations (I exported from one to the other, to be sure) - and disabling the firewall (just in case) doesn't fix it either.
I've tried online 'solutions' involving adding ProhibitIpSec and AssumeUDPEncapsulationContextOnSendRule to the registry, which then generates Error 800 instead of 789, so deleted them, again.
I've deleted and recreated the VPN settings a couple of times, and tried setting the properties to explicitly use L2TP/IPSec and (in combination with other tweaks) increased my score to Error 809.
I've tried wired and WiFi connections.
And overall I must have spent at least a whole day looking for ideas and trying different tweaks, without success.
The only other things I can think of are that it could perhaps be a driver error, or a bad windows patch.

But surely there must be some better way to log and diagnose what's going on, rather than just relying on an error code?
Or even a proven solution?

Help much appreciated...
 
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