Subject FB engine not finding F_LEFT

Perhaps my problem is in an incorrect installation setting? 

I have FB 2.5.3 on a laptop. Win 7 32 bit.

I introduced a stored procedure and in it call F_LEFT(:varName, 24) to truncate :varName from a vc(48).

This DB is used in Win 7 and win 8.1 else where without problem.

When I run the procedure on this one box I get 

ISC error 335544343

invalidrequest BLR @offset2452

function F_LEFT is not defined

module name or entry point could not be found

Error while parsing procedure myProc's BLR

I reinstalled Firebird.

I did a backup and restore of the DB. 

No improvement.

Any advice for me, beside "work smarter"?