If caseID and CustomerID are numbers then you can't do this:
(tblCustomers.CustomerID = @FilterText) OR
(tblCases.CaseID = @FilterText) OR

Since your FilterText is a string... when SQL encounters 12 (say for the CaseID) ... it tries to convert your FilterText (which is "h" ) to a number... "h" isn't a valid number and so the error is thrown.

-tg