Korganizer: "An error occurred while creating a new tag"

Hello,

I've had issues with to-do categories/tags for years.  Eearly on adding
a tag to a to-do item would duplicate it in the tag list and I regularly
had to go delete duplicates.  Now, even though I gather from the manual
that there are some preset tags/categories, selecting

Settings -> Manage Categories

brings up an empty "Manage Tags - KOrganizer" window, and when I try to
add one it pops up a "Failed to create a new tag - Korgznizer" window
that only says "An error occurred while creating a new tag", "OK".

My own categories don't show up either, even though they do
appear in the "Categories" column of the to-do list.

There is probably some corruption where my old categories are stored.
Does anybody know where this is?  My calendar is remote in a radicale
server.  Searching there, I see all the VCalendar calendar files and
they have what I would expect: the old ones have the CATEGORIES field,
the new ones (that I created while not being able to add categories)
don't.  Displaying the raw data with akonadi console shows the same.

This is under debian stable, up-to-date, with the following relevant
software:

mariadb	10.3.27
akonadi-server		4:18.08.3.7
akonadi-backend-mysql	4:18.08.3.7

In ~/.local/share/akonadi/akonadiserver.error I see:

Connecting to deprecated signal QDBusConnectionInterface::serviceOwnerChanged(QString,QString,QString)

And then this set of messages, apparently repeated every time I tried to
add a new tag/category:

DATABASE ERROR:
  Error code: "1452"
  DB error:  "Cannot add or update a child row: a foreign key constraint fails (`akonadi`.`tagtable`, CONSTRAINT `tagtable_ibfk_1` FOREIGN KEY (`parentId`) REFERENCES `tagtable` (`id`) ON DELETE CASCADE ON UPDATE CASCADE)"
  Error text: "Cannot add or update a child row: a foreign key constraint fails (`akonadi`.`tagtable`, CONSTRAINT `tagtable_ibfk_1` FOREIGN KEY (`parentId`) REFERENCES `tagtable` (`id`) ON DELETE CASCADE ON UPDATE CASCADE) QMYSQL3: Unable to execute statement"
  Query: "INSERT INTO TagTable (gid, typeId) VALUES (:0, :1)"
Error during insertion into table "TagTable" "Cannot add or update a child row: a foreign key constraint fails (`akonadi`.`tagtable`, CONSTRAINT `tagtable_ibfk_1` FOREIGN KEY (`parentId`) REFERENCES `tagtable` (`id`) ON DELETE CASCADE ON UPDATE CASCADE) QMYSQL3: Unable to execute statement"

A search for this error code found this:

https://bugs.archlinux.org/task/63025

But it is for a different distro and a more recent mariadb version
(10.4.6).  The workaround there is to downgrade to mariadb 10.3.16.
Since I'm running 10.3.27 I suspect it's unlikely that these minor
revisions are the cause of the problem.

And this:

https://forum.kde.org/viewtopic.php?f=215&t=124898

Which advises to rm ~/.config/akonadi/agent_config_akonadi_*_changes.dat

After doing that, the problem persists.

All the other references I found so far related to this error code are
years old.  I would hope the relevant bugs have been fixed.

I'm not familiar with the internal structure of the akonadi/korganizer
data, especially as it's stored in mariadb.  Is there a way to find if
my old categories are there and in a non-corrupted state? Where would
the default built-in categories/tags be?  What should I do next to try
to fix this issue?

Thanks.

Augustine