So your qubes clipboard stops functioning.
You can ++ctrl+shift+c++ and get the notification that it was copied to the global clipboard, but you cannot paste it with ++ctrl+shift+v++?
Do a sudo journalctl -xef in dom0 and try again.
For me there was a
qrexec: qubes.ClipboardPaste: sourceqube-> targetqube: error loading policy: /etc/qubes/policy.d/30-user.policy:19: wrong number of fields
Informing me, that i had broken my qrexec rules, which lead to the clipboard action failing to validate.
After fixing the rule in line 19 it worked again.