Question 5.2.8 -> 5.2.14 ‘Invalid extension update’

No difference.

Did you mean uninstall everything first?

The Dashboard shows odd things like Categories 9/0

Did you run diagnostics in Kunena Tools? The first digit is for the sections and the second for the categories. So there must be something wrong with your tables. Could be because you didn't update Kunena to 5.2.14 in your Joomla 3 site, but not sure.

Diagnostics shows the same error as before at the top of the page, but all the test show as PASSED.

Then I would start all over. I hope you did the upgrade in a separate folder. Then empty that folder, empty your database, restore a backup from your Joomla 3 site again, update Kunena from 5.2.8 to 5.2.14 en start the upgrade again.

Not even remotely practical. It’s disappointing that Kunena is so fragile, but in our case, with minimal content in the forum, recreating it would be the simpler option at this point.

The live Joomla 4 site just can’t be replaced at this point. People have registered; memberships have been paid. I would have to spend a lot of time fiddling about with Kunena in XAMPP and then export just the right bits. Not a productive use of my time.

In any case, I seem to remember that Kunena would not update in Joomla 3 either.

With all due respect, but if you didn't even keep your Joomla 3 site up-to-date you shouldn't complain about the upgrade. If you do all your tests on localhost and make notes of the steps you took, it shouldn't take much time to then upgrade your live site. I've upgraded a site with about 10.000 users and about 150.000 posts without problems. But I did make sure that all my extensions on my Joomla 3 site were up-to-date or deleted/replaced if they didn't work with Joomla 4.