After trying i can say, the model does not converge if NLgeom=ON because of the connections between rigid body constraints and shell elements. It can be seen in the picture, the simulation runs successful when fixing the lower edge, but does not converge when fixing the upper edge. (The model has no contact only tied- and rigid body constraints).
If these two flanges where made of solid elements, i am sure the nonlinear analyses would converge.
