Answer:
First, you should install the latest version of RFEM 5. The current versions are: RFEM 5.20
Then, replace the RFEM64.exe.config with the file attached here in the FAQ. This file can be found after the standard installation of RFEM and in the following folder:
C:\Program Files\Dlubal\RFEM 5.20
Then, run DLInstaller. You can find this file under the following path:
C:\Program Files (x86)\Common Files\Dlubal\DLInstaller
After restarting the programs, the import and export to Tekla should work again.