"not responding" + "your score cannot be saved the system cannot find the file specified."

As stated in the Subject line:

1. As a long time finale user now in refugee status I gravitated towards Musescore.
   My first project was a string quartet ~450 measures.
   While entering the score i received repeated "not responding" interruptions to the point where they
   took more time than note entry.

2. The next day I opened the file to do some editing and make parts. The program gave me the second message above "your score cannot be saved....."
   The program successfully opened the file from my computer but was not able to save edits.
   I tried multiple approaches: opening the auto-backup file; closing without saving and re-opening (successfully). Nothing helps. I cannot save anything.

File attached below.