Next step should be disable all the plugin via adminCP.
Undo the config.php edit.
Enable one plugin at a time (via adminCP) and test the forums, the plugin causes the problem after enabling it, is the culprit.
Thanks. Plugins are disabled via ACP (and most were disabled individually from plugin Mgr.). However, the problem is not solved for the majority who have a problem, which is that they cannot log in. So I don't see what re-enabling plugins will do.
I went back through email and PMs and it looks as if this might have started when we installed Glow Host (back on v. 4.2.2). That's been disabled for months, but could it have caused a change in code someplace?