“There was an error changing your username”
I want to change my username, but when I try, a prompt pops up saying 'There was an error changing your username.' It's been several months since I last changed my username. Why can't I change it?"

From the forum guidelines you were linked to: Blanking out your thread isn't some clever hack around this — it's just vandalism. I've restored this thread.
Someone will investigate your issue and respond here when they're able to do so.
There is generally no reason you should need to do this. We allow changing your username so you don't need to delete an account and break syncing just because you want a different username. You should only delete your account if you do not plan on using Zotero again.