Hello,
it seems to me that the model now contains objects that are connected to each other not via a beam or a surface, but via simple lines. These have no stiffness and therefore cause the singularity.
One method to investigate the instability is to perform the stability analysis with the settings shown in Image 1. This will provide you with a deformation pattern that can give clues about the location.
Kind regards
Eike Hartmann
Image 1:
