Most probably you have an error in one of your policy files in /etc/qubes/policy.d/ in dom0.
Did you edit any policy files?
Run this command in dom0 terminal:
journalctl -f -n0
Then try to paste the text from the Global Clipboard to any qube and see if it’ll give you an error about qrexec policy in the journalctl output.
Maybe it’s because of this: