LoCalc crashes when saving one spreadsheet

I have a localc spreadsheet under Ubuntu 20.04 which has worked without any trouble for several years. Suddenly it has taken to crashing localc whenever I try to save it, whether I have made any changes after loading the copy on file or not. Other spreadsheets pose no problem and are recovered normally when I restart after the crash. The problem spreadsheet is also recovered normally after the crash, but if I immediately try to save it, localc crashes again (and the copy on file is unchanged). I have tried restarting the computer but that makes no difference.

What can I do to track down the problem?

localc --version gives:
LibreOffice 6.4.7.2 40(Build:2)

Please test in safe mode, Menu/Help/Restart in Safe Mode

Also consider updating to a newer LibreOffice version, your 6.4.7 is almost as old as your Ubuntu 20.04. Available from the LibreOffice Still PPA is 4:7.6.7-0ubuntu0.20.04.1~lo1

Still crashes in safe mode.