Parallels 12 using Windows 10 graphic distortions

Hello community,

I am using macOS 10.13.2 and Parallels 12 (recent update) and am running on an MacMini Server (Mid 2011) with an i7, 16GB RAM, 2x 1TB SSD, 1 Thunderbolt Display (2560x1440) and one 27" Display Port LCD (FullHD) running Parallels in Fullscreenmode.
I was able to work normally with everything within Windows 10 but all of a sudden I encountered heavy graphic distortions which occur mostly when using the Snipping Tool from Microsoft or when Pressing "Windows-Key" + "Tab", sometimes when switching between active windows.
The strange thing with snipping tool is when I open it up, all is normal but as soon as I hit the "new" button to get the selection cursor the whole screen content seems to be mixed up with the content of the other screen.




I already tried the following:
* Disabled all Win10 Effects
* Increase Parallels GPU RAM (256->1024MB)
* Reinstalled Parallels Tools for Windows
* Rebooted the VM
* Installed all available Windows 10 Updates

While writing this I tried something different which seems to work so far ... I reduced the resolutionof the Thunderbolt Display to match the FHD resolution of the 27" LCD, but I'd rather not keep it that way (blurry).

Thanks in advance.

 

I would be thankful if someone could try to verify if it happens for them too while using two LCDs so I have a confirmation.