Preben, We have run across this occassionally. Although we haven't determined the exact cause we have found a workaround. The Check In is trying to build the WTPart. You can troubleshoot for the specific WTPart by Checking In using a "bottom up" approach with the assembly. Check In the Parts first then work up to the assembly. When you find the suspect object that won't check in then disassociate the WTPart from that CAD Doc. Check In the CAD Doc then re-associate the CAD Doc and WTPart and Check In both.