update in case it helps anyone
the backups may be working again
for backups - for Wordpress installations but perhaps other apps as well, there is what I believe is a new option for the backup location: a generic 'your hosting service'. If that is selected, a manual backup appears to work just fine. Another new storage location though

. This time a directory is created under the www directory.
no response from Installatron yet, my last ticket from August is still open - I happened to notice the new option while doing something unrelated, and tried it just for grins.
I didn't verify it by doing a restore ... I'll let someone else do that -- so all I can vouch for is that it completed happily - didn't generate any errors and doesn't appear to have caused any problems - and it created a new file that is about the right size compared to other backups just completed