Created a spreadsheet, typed some data into it. Came back the next day to save the file. Neither Save nor Save As... will work—the beach ball spins for a few seconds and nothing happens. I copied the data into a new spreadsheet; same story. Wound up copying it into a Google sheet.
Steps to Reproduce:
1. Create a spreadsheet
2. Type some data into it
3. Wait a day
4. Try to save the file
Save and Save As... both just show the beachball spinning for a few seconds and nothing happens. No File dialog.
File Save dialog appears and allows picking a location to save.
Reproducible: Didn't try
User Profile Reset: No
[Information automatically included from LibreOffice]
[Information guessed from browser]
OS: Mac OS X (All)
OS is 64bit: no
Could you try "Grant LibreOffice Full Disk Access in Security & Privacy -> Privacy in System preferences" for a workaround?
Also, you can upgrade to 6.4.5 just to have last version even if the pb hasn't been fixed yet.
*** This bug has been marked as a duplicate of bug 128233 ***