Project Manager hangs & progress bar halts without specifying which (if any) file is causing the error

This does happen...

Have you tried exporting the same sequences that you are looking to project manage as a standalone Premiere Project first?

Then from this new project it should have cleared out any unused files. 
Then check after your out-points in each sequence for any leftover clips that could be hangovers.
Check if you have proxies attached perhaps and if they are contributing to it failing.
You should also be able to clearly see if there are any outlier file types in the project.

I’m having the exact same problem. No cloud drives. Did you find a solution?

Try creating a new project and import the old one into it.

Then try the Project Manager.