Answer:
Between RFEM 5 and RFEM 6, compatibility is only ensured in the direction RFEM 5 → 6. In RFEM 6, you can open RFEM 5 models without specifying add-on modules. Unfortunately, the models cannot be opened in reverse.
In the older program generation, the add-on modules were separated from the main program, while in RFEM 6, the new add-ons are fully integrated into the main program.