CardDav addressbook stuck at "Fetching collections (0%)"

Hello everyone,

This has been a recurring problem.  It first appeared on Debian 9 and there
was nothing I could do get akonadi/kaddressbook to connect to the Radicale
server.  I eventually gave up, upgraded that system to Debian 10 (current
stable release) thinking that it might be a bug in old software, and turned
it off.

The new system (different computer), Debian 10 completely up-to-date,
worked fine for a while but now I'm facing the same issue.  In the
meantime, DavDroid on the cell phone talks to the server just fine.
In an attempt to debug the issue, I turned on the old computer and
started Kaddressbook and Korganizer.  The addressbook and calendar
synchronized just fine, surprisingly.  Updates from there do propagate
to the server.  Just after the upgrade, "Fetching collections" was still
stuck, IIRC at 42 or 47%.

I have tried removing the offending addressbook repeatedly on other machines
where I've seen this problem, but the only result is that akonadi only
increases a count and I end up with resource_5, resource_6, etc...

In the present situation, the CardDav addressbook is stuck at
"Fetching collections (0%)" and the CalDav calendar doesn't show in the
KOrganizer Main Configuration -> Calendars, instead there is another
entry called addressbook and this one says "Ready".  This is clearly
incorrect.  Either KOrganizer/akonadi has mis-labeled the calendar (more
likely) or it thinks there is some other remote addressbook that doesn't
exist.  On the Radicale server there are only addressbook.vcf and
calendar.ics.

On startup of Kaddressbook and Korganizer, even after stopping and
restarting akonadi (via akonadi-console), the calendar/todo list and
addressbook display properly, but this might be the local copy.  Any
modifications/additions don't propagate to the server.

Has anyone else encountered this issue?  What can I try to get akonadi
to complete the fetching of collections?  I'm hesitant to try something
I don't fully understand as I don't want to risk the server synchronizing
to an incorrect state and losing all my data.  Akonadi-console is very
confusing due to the loads and loads of data and not being familiar with
the internal protocols.

Any help greatly appreciated.

Augustine