Large Assembly: Unable to solve

Hello,

We have a large assembly (structural steel frame) including a lot of steel beams and bolts/nuts. The assembly contains fully defined connections and constraints.

After solving the analysis we got the message:
“Unable to solve equation system (status 17, 4276, 0). The structure may not be sufficiently constrained. Do you want to check possible rigid motions?”

After directly checking Rigid motions we get the message:
“The structure is sufficiently constrained. No rigid body motions was found.”

These are conflicting reports. Now i have to check manually every part and connection in the large assembly.
Do you have some tips? Thanks.