clipboard not working after Security Update
Solution 1:
I had a similar situation, though it was caused by a sudden power-down in my case. Copy did not work in any of the apps I tried - viewing clipboard showed it to be empty. Restarting or logging out and logging in did not help. After several hours I stumbled onto an answer, which helped me.
In short, run in Terminal command, which rebuilds a cache of shared system services.
sudo update_dyld_shared_cache
Probably I restarted my Mac after that.