Answer:
This problem occurs when the imported cross-section description does not exist in RFEM/RSTAB. In such cases, it is possible to create a kind of "translation table" in the form of a conversion file. You can find this in the "Detail Settings" tab of the "Import" dialog box (see the image).
The program then converts all entered cross-sections during the import so they are recognized by RFEM/RSTAB.
A similar conversion table is also available for materials.