I've been facing the same issue! I can't copy any table nor any cell from a single Numbers file! Doesn't matter what part of this file or wherever I want to copy into, it simply doesnt work!
This file is NOT LOCKED!
The only "new procedure" I've performed has been to send it to be edited in my iPad. I'm not sure, but maybe sharing this file with iPad has damaged it. I wish I find out later that I'm wrong, but right now I wish I hadn't shared this file with iPad.
I guess I'd better export my file to MS Excel and continue my work there from now on.
Anyway, I'd be glad to understand what happened and to learn how to fix it!
Await for replies and contributions.
Regards, Ciro.