Strange Windows 10 VPN connection issue

I have a strange connecting issue with a Windows 10 machine. The serves is on 9.510-5 and has an intel I350 adapter, the PC Win10Pro 1803, it has an intel I218-V nic. I use L2TP over IPsec with a preshared key.

If I boot up the PC and try to connect immediately after the start everything works perfectly, I can login and the connection is stable. If I use the PC for a while, have some programs running and then try to connect I can’t, the login status simply returns to the default. In Windows event viewer there is only one related line:

The user xx\xx dialed a connection named VPN which has terminated. The reason code returned on termination is 829.

In UTM’s log this is what I see when the connection is not working:

 

2018:08:28-13:00:16 xx pppd-l2tp[6729]: Overriding mru 1500 to mtu value 1380

2018:08:28-13:00:16 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:19 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:22 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:25 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:28 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:31 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:34 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:37 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:40 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:43 xx pppd-l2tp[6729]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x6a3e8975>]

2018:08:28-13:00:46 xx pppd-l2tp[6729]: LCP: timeout sending Config-Requests

 

and this when I can connect:

 

2018:08:28-13:03:06 xx pppd-l2tp[6983]: Overriding mru 1500 to mtu value 1380

2018:08:28-13:03:06 xx pppd-l2tp[6983]: sent [LCP ConfReq id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x7977bf4e>]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: rcvd [LCP ConfReq id=0x0 <mru 1400> <magic 0x4f780293> <pcomp> <accomp> <callback CBCP>]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: sent [LCP ConfRej id=0x0 <callback CBCP>]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: rcvd [LCP ConfAck id=0x1 <mru 1380> <asyncmap 0x0> <auth chap MS-v2> <magic 0x7977bf4e>]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: rcvd [LCP ConfReq id=0x1 <mru 1400> <magic 0x4f780293> <pcomp> <accomp>]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: sent [LCP ConfAck id=0x1 <mru 1400> <magic 0x4f780293> <pcomp> <accomp>]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: Overriding mtu 1400 to 1380

2018:08:28-13:03:06 xx pppd-l2tp[6983]: PPPoL2TP options: lnsmode tid 60248 sid 17944 debugmask 0

2018:08:28-13:03:06 xx pppd-l2tp[6983]: sent [CHAP Challenge id=0x96 <7262bfeba167acccc95ef6126>, name = "xx"]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: rcvd [LCP Ident id=0x2 magic=0x4f780293 "MSRASV5.20"]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: rcvd [LCP Ident id=0x3 magic=0x4f780293 "MSRAS-0-xx"]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: rcvd [LCP Ident id=0x4 magic=0x4f780293 "\031\n$F\354K\211O\231u\204;\241\003\217\337"]

2018:08:28-13:03:06 xx pppd-l2tp[6983]: rcvd [CHAP Response id=0x96 <4b100c526033fd3a5200000000000000d8033869df4fd2ba7ca12868dff2fe5c9be8cea704d4adae00>, name = "xx"]

2018:08:28-13:03:08 xx pppd-l2tp[6983]: convert_to_utf8: is already utf8 - nothing to do here

2018:08:28-13:03:08 xx pppd-l2tp[6983]: AUAconnect - Returned IPAddr *

2018:08:28-13:03:08 xx pppd-l2tp[6983]: AUA: authentication of user xx succeeded

2018:08:28-13:03:08 xx pppd-l2tp[6983]: sent [CHAP Success id=0x96 "S=693DEC8A74731AE7DEBB81 M=Access granted"]

 

Have anyone experienced something similar? Thanks in advance