I suddenly get this strange error when trying to run a stored procedure in Script Executive in IBExpert or the script editor in Workbench.

The script I am trying to run is
CONNECT 'C:\KIOSKDB.FDB' USER 'SYSDBA' PASSWORD 'masterkey';
execute procedure sp_cart_add(24,'RENT',43);

And the error I get on the second row is:
"Unsuccessful execution caused by system error that does not preclude successful execution of subsequent statements. message length error (encountered 0, expected 8)."

I Database workbench I also get this additional information
< ERROR:
ISC ERROR CODE:335544358

ISC ERROR MESSAGE:
message length error (encountered 0, expected 8)

STATEMENT:
TIB_DSQL: "<TIB_Script>.<TIB_DSQL>.">

I have no idea about what any of the errors mean but I don't think it is the SP that is eronous because it works if I run it as a single SQL statement from the SQL editor. I have run it in the script editor before without any hickups. Can the database have become corrupt in anyway that is not noticable elsewhere? I have tried to backup and restore it but it didn't help.

I don't even know where to look for a solution to this so any pointers will be much apreciated.

Tim