Z-Table remains unchanged: We want to use a function module which updates our own Z-table. This function works fine in the SE37 test environment but when we call it from a GuiXT script the database remains unchanged. What could be the reason?

You need a "Commit work" statement at the end of the function module. The commit is not performed automatically at the end of a function which has been called via RFC. Alternatively you can call "BAPI_TRANSACTION_COMMIT" from your script.