Answer:
The *.dll and *.tlb files were probably not updated correctly on your computer. Please proceed as follows:
1. Rename the following folders in Dlubal.bak:
C:\Program Files (x86)\Common Files\Dlubal
C:\Program Files\Common Files\Dlubal
2. Reinstall RSTAB.
3. Move the files from the newly created Dlubal folders to the respective Dlubal.bak folders (overwrite all).
4. Rename the Dlubal.bak folder to Dlubal.