Yes, you're right. Debian works (even with GNOME), XFCE works, MATE, Windows, ... it's just Ubuntu with GNOME. Sometimes.
And yes, I know PVE is a server virtualization platform. I just wanted to say: Of course you can switch from SPICE to VNC, if that (sometimes) circumvents the problem, but you might have a reason for using SPICE. And it would be nice if the dots are a bug that someone can fix.