It's going to be very difficult for us to help without the files, or more information. I assume this only happens with these particular files? This may be a problem to take to your VAR.
I have a fairly complex multi-level assembly. I can open the top level assembly OK, but when I try to open the mid level assembly the program crashes. If I edit the assembly from in the top level, and suppress all the sub-assemblies inside it I can then open it. After unsuppressing components one at a time I discovered which sub assembly was causing it to crash. however I can open that lower level assembly independently and there are no apparent issues. I tried deleting the sub assembly from the mid level and re-inserting it to maybe change IDs but that didn't work either.
I would suspect;
- Same named files with a way different profile that drives another part
- Same reference files (such as renaming a file and it still is linked to the exact location)
- Really bad circular rebuild issue