That error is actually being returned from your MySQL server. not UBan itself. UBan just forwards it on to you when it gets the error from your SQL server.
I don't know enough MySQL to offer where to look, what settings to check, etc, but if you just started having the problem, I'd look into what all you have running from your MySQL server, and verify no other problems exist. Or if your setting for maximum connections is low, raise it a few.
If it's already pretty high.... I'd investigate it possibly being exploited/DDOS'ed