Well, to figure out what might be going on here, I'm going to need you to give me more information. First, I need you to give me the version numbers of each relevant item:
Joomla Version
JoomlaBoard Version
JoomlaBoard SearchBot Version
Also, It would help to have the Debug information for the failed search.
First, turn on Debug in your system configuration.
Second, perform a search that would normally return the error you have been getting.
Third, copy and paste all of the debug information into a personal message to me.
PM Me
This will get me the information I need to help you fix the problem but won't release potentially sensitive information to the public. Be sure that you turn debug OFF after you copy the information needed.
Thank You,
vujsa