First, please let us know what's happening, so we can identify if a bug is the cause.

Make a copy of your Libraries folder (your error message shows where you can find it) and save it someplace safe.
Go in to the original Libraries folder, and delete all the files with the extension *.lib; if your computer is set up to suppress the extension, it will help to change the View to Detailed mode.  You can then sort the folder contents by the Type column, and delete the  files of Type "Object File Library".  Leave the Definition files for now.
Try starting REM again;  if it starts, you'll find that all your libraries have reverted to the default content.  If it doesn't start, then make sure REM is shut down and then delete the Definition files (the *.def files).
If you end up deleting both the *.lib and *.def files, you'll need to run the REM install again, to create a working library.  The reason why reinstalling did not work before is that REM does not overwrite an existing library, to protect you from losing all the library entries you create over time.

Once you have a working copy of REM again, you should create a backup copy of the "good" Libraries folder. 

If you want to recover your old libraries, you can experiment with shutting down REM and replacing individual *.lib files from your original (non-working) Libraries folder backup to your active Libraries folder, then restarting REM.  As long as it starts, the replaced files are usable.  If it doesn't work, then you need to restore the most recently replaced *.lib file with the working copy from your "good" backup of the Libraries folder. 

The other way to restore your libraries is to open old building files that used those libraries; when REM prompts you, accept those library entries into your own library.