Hi again and thanks for the info.
We've tried the tcp mss fix but it does not help.
the strange thing is that when connecting from internet to the same server via RDP(dnat) all is fine connection is stable and does not disconnect.
just upgraded the srx to newest version 12.3X48-D25.3 from 12.3X48-D20, rebooted not helping.
tried to disable alg on msrpc and addind "security flow tcp-session no-sequence-check" something I found on the web.
this is what wireshark find when connecting to rdp from client at lan1-srx-leasedvpn->server at lan2
as stated connection from internet works just fine to same server.
we are also experiencing problems with printing from server at lan2 to printer at lan1
so it seems like some communication is blocked or messed with between the lans,
and again switching back to sonicwall solves everything...
do you have any more ideas?