I found on Debian with on KDE Plasma 5.12 that clipboard is constantly freezing when virtualizing virtual machines. I tested this behaviour both with Debian VM with Plasma and Windows VM. To reproduce this issue simply install and upgrade host to Buster and run one or more VMs with "spice-vdagent" that handles clipboard. Simply copy text (Ctrl+C/V) from host to host or from host to VM or from VM to host or within. In random intervals paste fails and time to time keyboard input hangs so you cannot even type. The solution is to close virt-viewer/virt-manager. Then everything is OK again and you can copy and paste. Bug was initially filled in Debin bugzilla: https://bugs.debian.org/cgi-bin/bugreport.cgibug=901655
The link in previous post does not work. Bug in Debian bugzilla has number: 901655 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901655
*** This bug has been marked as a duplicate of bug 360262 ***
*** Bug 301505 has been marked as a duplicate of this bug. ***