You say that the error is on a certain line in a web script where there is a call to a stored procedure which is in "another database".

But is the error with that call itself? Or does the procedure start executing and then runs into this error? An error message from SQL Server includes both procedure name and line number. If the line number is 0, the error is with invocation of that procedure. But of course, if you are looking at this from the web application, the web application may careless drop the information on the floor.

One way to investigate this is to do:

EXECUTE AS LOGIN = 'WebScriptUser'

go

EXEC storedproc @par1, @par2, ...

go

REVERT

This should give you a clearer error message you can work from.