CAESAR II lineraizes the non linear system ( if so) before running the dynamic analysis. It may have so happened that for the particular static case chosen, some supports may not be there during linearization as they may not be load taking. This may have created a system where the stiffness matrix is singular.

Try to select other cases for " system linearization". This may give you the desired stable system.Friction is also a non linearity which the dynamic solver cannot handle. May be the friction stiffness inclusion in the dynamic solution can help eleminate the rigid body modes.

Anindya Bhattacharya