[GNC] Error msg (when trying to save): check permissions or sufficient space (neither of which make sense)

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Hi there,

I'm using GnuCash Version: 5.2 (Build ID: Flathub 5.2) on Linux.

The last time GnuCash properly worked for me was in early May; now,
whenever I try to save changes (or save as new file, or copy the *.gnucash
file elsewhere and save changes there) I get the following error message:

*msgid "Could not write to file %s. Check that you have permission to write
to this file and that there is sufficient space to create it."*

Google only returned this link so far, where that error message features:
https://lists.gnucash.org/pipermail/gnucash-changes/2017-December/015461.html

Any clues / ideas how to get unstuck here?

Many thanks!

Harry