Did you apply opposite moments (one positive and one negative) and did you use the same simple supports ? I’ve tried it yesterday and I think it wasn’t so bad.
Where is this long quote from ? I can’t find it when I search for the exact strings.
I would say that symmetry, rigid body constraints, buckling or nonlinearities and shells indeed are problematic combinations, especially in CalculiX due to its significant limitations. It’s often better to avoid rigid bodies, sometimes even symmetry (particularly for buckling problems), but more importantly, it often makes sense to use solid elements (at least for part of the model, e.g. where rigid body constraint is applied).
Yes, it’s a major limitation of CalculiX. As I’ve mentioned, its dev decided to prevent the usage of rigid body constraints with shells even in linear analyses in the newest release of CalculiX (not yet provided with PrePoMax), but I asked him if he could allow it for linear cases.
Here’s a list of the most important known CalculiX limitations: Known CalculiX limitations