well i tried deleting a user and this is what i get
Hes the only one that this happens to....
You try deleting a user an you get something about private messages? Umm do maintenance an try repairing your tables. Not sure of this one. Give a try.
Try running:
REPAIR TABLE mybb_privatemessages;
In your myphpadmin, it usually fixes it for me.
ACP -> Tools & Mainteinance -> Optmize Database -> select all the tables and click ok