Problem with connecting to servers

Hi all,
Using RealVNC (Windows 10 pro) for a long time and it allways worked well. Recently I had reset my router, and now it takes a long time to connect to other servers (Windows 10 pro). Initially 3 quarter of the screen remains black. Clicking on something hardly doesn't work. An exception is when I connect to a linux server. That works flawlessly.
What can be wrong?
Thanks beforehand,
Fokke Nauta