Answer:
Please check the definition of the scissor hinges in the RSTAB 8 file.
To properly consider the function of a scissor hinge, the scissor hinge of the same type must be specified on the member ends of both members "touching" at a node. If this is not the case, you get the mentioned error message.
The described error message should prevent problems arising during the calculation: The structure could become unstable due to the incorrect hinge definition (already in RSTAB 8).
Therefore, the problem should be adjusted either in the RSTAB 8 file or in the RFEM model after the import.