> CREATE TRIGGER UPDATEPROPID_IMPROVEMENTHEADER FOR IMPROVEMENTHEADER
> ACTIVE
> AFTER UPDATE POSITION 5
> AS

...

> EXECUTE PROCEDURE CheckAsmtCode NEW.AsmtCode1,
> NEW.PropID, NEW.ImprType_Code, NULL, 1 RETURNING_VALUES NEW.AsmtCode1;

You are assiging value into NEW.AsmtCode1 in AFTER UPDATE trigger which is forbidden in FB2.

Ivan