Drop File Error

I'm working on a large assembly with several large sub-assemblies inserted, ~4200 parts total.

Recently I'm getting a Drop File Error pointing to 'default.sldprt', been using SW for ~10 years and don't remember his error.

I tried renaming the file in WinExplorer but SW recreated the default toolbox part when I re-opened the assembly, then gave the same error message.

Any thoughts on where to/how to correct this?

Thanks

SolidworksToolbox