Answer:
Rib members are available in RFEM for modeling downstand beams, ribs, and T‑beams. These supporting structural units always consist of a flat plate section and a supporting cross-section that is placed eccentrically to the plate. Together with the specification of the eccentric supporting cross-section, the rib member combines the entire unit consisting of a flat plate cross-section and a member-shaped supporting cross-section with the results of internal forces depending on the effective slab width for the entire package.
Error Message No. 1663 appears if there is at least one member with the "Rib" member type that cannot be assigned to the connected surface. This situation can be caused by the copying process with the additional option to create new members between the source nodes and the newly generated nodes on the basis of a template member with the rib definition.
In this case, you have to change the member type of the affected members from "Rib" to "Beam".