here are some steps I might use, some of these may not be needed since you have a mesh, but there is a possibility something tips you off to where the problem is.
make each part and assembly the same absolute accuracy in model properties.
look at "geom checks" to see where the geometry problem might be and correct.
Confirm mesh each part separately with the correct element size.
Check for tiny mismatch between parts in assembly and preferably check for and remove any interferences
check that mating surfaces are actually mated and re-mate in assembly to confirm.
Turn on slowly each component in the assembly and confirm mesh / solvability.
Check/confirm bonded / free / contact surfaces are as expected as you do so.
If problem persists do a binary search for the issue by cutting the model in half or cutting out portions of the model and rerunning to narrow down where the issue is located.
On an extremely rare case I have had to export the model as a step file and import it in and redo the setup because there was some issue in the underlying data, but that is for stuff that was imported or "dumb" geometry to start with and very occasionally import data doctor will mess something up to the point of no recovery. You could comment on whether the frame is native Creo or components were imported or have really no model tree.
-Regards
