Answer:
There can be many reasons for this. Your model may have tension members and you did not deactivate the "Connect Crossing Members" option during the import.
Your model is unstable at the crossing points of tension members.
If I deactivate this option, I still get an error message. What can be the cause?
The calculation parameters were probably not adjusted sufficiently in the file previously. Tension members often require special treatment, which can be added in the calculation parameters.
Further information for learning the reasons for the instabilities is described in this FAQ.