[_ck_]

“use display name” is notorious for causing disappearing forums and other behaviours though it could very well be another plugin.

#1 rule for diagnosing a problem is to slowly turn off all plugins and see which one is causing it.

A bigger problem is that a plugin could in theory even interfere with the creation of an account, permanently damaging it by not giving it the proper access rights to start with – that can only be observed by looking at the tables, ie. phpmyadmin.