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Buckling can also occur in horizontal floor elements  such as beams and floors, if the floors take in plane loads (i.e have no diaphragm or semi rigid diaphragm) and have insufficient capacity, due to structural properties or small stiffness  modifiers. Using  rigid diaphragms and checking if the analysis converges can help rule out this as the cause of the problem.

Additional suggestions which may resolve negative-stiffness errors include:

• Remove auto line constraints. Select all deck/shell objects in the model, then select Assign > Shell > Auto Edge Constraint , check the Do NOT create Edge Constraints, and uncheck the Apply to Full Structure (not just Selection) option. This will remove auto line constraints and possibly resolve instabilities.