I have MT RB1200 (v6.20) as VPN server in office, and hAP lite (v6.40) at home (dynamic IP) with L2TP tunnel in between.
Everything I do in company's private network (browsers, ssh, ftp, ...) works fine, VPN connection is stable for days and weeks except Microsoft RDP (Remote Desktop Connection Manager v2.7). i don't use much RDP, have only a few Win servers there, but from time to time I need to use it.

Every time I connect via RDP to any Windows server in company's private network after max 2-3 minutes VPN connection droppes down and VPN then reconnects. Network connection is not affected (I have ADSL router in front of MT), just affects VPN. If I use tunnel directly from my Win PC to same MT VPN server all works fine, no problem with L2TP, so I guess something with my local hAP lite settings is not ok.

Log on VPN server side: l2tp-out1: terminating... - hangup
Log on VPN cient side: l2tp-out1: terminating... - session closed

Tried L2TP tunnel with or without ipsec, same thing happen.

Any idea what can be a problem?
Thank you in advance for any help.

I had the same issue. As far as I can rembember it had to do with mangeling the MSS value.
In older firmware by firewall/mangle rule, today by a checkbox in ppp/profiles

So first check on the server: is „change TCP MSS“ enabled in the used profile.

And second try disabeling fast path on VPN connections.

I run 6.41.3 (stable)...

Ralf.

Yap, disabling fast path did the job.

Thank you Ralf