It appears to be something to do with the file, I just tried exporting some of the sub-assemblies and had no issue.
Change your views from lightweight to fully resolved.
The feather on the view icon indicates a Lightweight View. Not all the data is loaded for lightweight views to help with drawing performance.
1. Open the assemble file and just rebuild file file.
2. Might be the reference of a drawing file from assemble may missing. That's you may change the file path of an assemble file.
1. File -> Save as -> Set file type ".dwg" _> specify name _> Click "ok".
View the attached file below.