well. thanks to the last tipp I can format my pc now. It keeps disconnecting my RDP every 5 seconds. Even after switching back. flawless working for 3 years and now i got this. got enough tips for now. thanks anyway.
AcidDream
on remote computer:
Open
Group Policy Editor (gpedit.msc) Go to:
Computer Configuration -> Administrative Templates -> Windows Components -> Remote Desktop Services -> Remote Desktop Session Host -> Connections
Open "Select network detection on the server" , click "Enabled" and choose "Turn off Connect Time Detect and Continuous Network Detect" from the dropdown menu under the "Select Network Detect Level"
Restart the remote machine and the issue should be solved.