RDP sessions dropped

We have switched from our old Fortigate 100A to a bigger one, but we are suffering some annoyances. Idle RDP(5.2) sessions are being dropped after a few minutes, despite of 3600 seconds set by default with session-ttl (or whatever we set). It happens after minimizing mstsc.exe window for a few of minutes (more likely depending on high connection latency). we tried some workarrounds, but they did not solve the issue (Fortigate: set timeout-send-rst enable or Terminal services: "send a keep alive packet every 1 minute").

Any help would be appreciated. Thanks in advance,

Juan