[GNC] Reports blank - after some account changes?

[Nick Meyne]

Hi There.

I'm on Ubuntu, Gnucash Version: 4.4, Build ID: 4.4+(2020-12-28) -  a newbie
to Gnucash, and was getting on just fine until...

All the reports in my gnucash accounts have become blank.  A tab or a
window is created, but nothing is visible, not even a 'no data'
message, even though the transaction and balance data is there, and
seems OK.   I have tried the account period selection values and they seem
OK - i did nothing to change them, and they still look OK to include
the data.  Is there something else that may be causing this problem?
I made some account hierarchy edits (removing unused accounts) but nothing
to suggest an error or corruption or incompatibility with the reports.

Is there a bug?  I saw there was a very old one.....

Any suggestions?

Nick