Answer:
For successful import, the main program RFEM or RSTAB must be registered in Windows. The registration can also be done manually if problems arise. To do this, run DLInstaller.exe with RFEM or RSTAB closed and right-click → "Run as Administrator".
By default, this file is located in the following path:
C:\Program Files (x86)\Common Files\Dlubal\DLInstaller
Then, go to the "COM" tab and select the version used (see image). To start the registration, click the "Register" button. Now, you can try to export/import again.
