It's probably the bypass flood check permission. Be careful with this though. Unless you have an addon editing it, I thought the timer was set for 30 seconds not 3 minutes.
You really don't want their primary usergroup to change at all. Primary usergroup should be set to registered permanently, any added permissions should be used as secondary usergroup.
When you upgrade your forum and all of sudden nothing works. Like the hundred or so threads that came about after 2.3 was available wanting to know why they can't access their forum. :ROFLMAO::ROFLMAO::ROFLMAO: