Windows – RDP session Flickering – Only one user

So, This is a tricky one.

I use a system where users connects via VPN to a Terminal Server on an other site. It is working fine for everybody but one user. For this user the screen is flickering/flashing/refreshing constantly and the session is unusable. It did work two days ago. No changes know by the user since then. Computer in Win10 and Server 2016

What I did is:

play around with caching, resolution, color depth (in mstsc settings) ... > same flickering
erase the remote desktop cache > same flickering
log with mstsc on an other server with his computer > works fine
try to connect with his user on my own computer > same flickering
download the other remote desktop from microsoft store > same flickering
try to update network card driver, video driver > all up to date
windows update > same flickering
check network settings > everything is fine
properly sign out the session of the user on the server > same flickering
erase the user profile from the server > same flickering

Performance of the PC are all good. No overload network, ram or cpu

So I am clueless…any other ideas woudl be greatly appreciated.

Ben