RDP disconnects

Hi, could someone help me with my issue regarding rdp session disconnects? We recently replaced our old fw with fortigate unit most of the stuff works fine, but there is one annoying bug. When connected via vpn, rdp session gets brief(5-10 sec) disconnect every 5 minutes. I'v spend many hours trying to find the cause. I have ticket with with their support, but after 2 months of "investigating" there is pretty much 0 progress, might be even worse after messing with various settings(ttl,mss etc..). The RDP work perfectly fine on local lan or with our old vpn system.We are using pretty much only MS products Win10 and Win2k16. I have ipsec,ssl and l2tp vpn setup on fortigate but its the same problem on all 3.

This are packets when the rdp session drops: "15196","14:38:35.313416","192.168.2.10","192.168.2.3","TCP","123","3389 → 25779 [PSH, ACK] Seq=5638053 Ack=110353 Win=63294 Len=69" "15248","14:38:37.366950","192.168.2.10","192.168.2.3","TCP","1354","[TCP Retransmission] 3389 → 25779 [PSH, ACK] Seq=5636988 Ack=110353 Win=63294 Len=1300" "15326","14:38:42.167267","192.168.2.10","192.168.2.3","TCP","1354","[TCP Retransmission] 3389 → 25779 [PSH, ACK] Seq=5636988 Ack=110353 Win=63294 Len=1300" "15477","14:38:49.292559","192.168.2.3","192.168.2.10","TCP","60","25779 → 3389 [RST, ACK] Seq=110353 Ack=5636988 Win=0 Len=0"

Any help, suggestion appreciated

Thanks