Abaqus: What is the mistake attaching connector elements to discrete rigid shell elements here?

[JWChen]

I am modeling a simple suspension system (see figure below). The two bodies are modeled with discrete rigid shell elements. A point mass is added to the reference point of the upper rigid. The lower cube is constraint in all 6 DOFs at the bottom. 12 axial connectors are defined between the two bodies to model some springs.
However, when I tried to conduct a modal analysis, I got the following error:

Three factorizations in a row failed. Check the model. It is possible that the model contains the kinematic coupling definition set up in a way that a degree of freedom has neither mass nor stiffness.

It seems that my mass matrix or the stiffness matrix is wrong. Would someone kindly point out where I made a mistake? Is there a tutorial involving connectors with rigid elements?

The input file is also attached. Many thanks.