So, regularly uploading new epubs to my Calibre library, then marking them for synchronisation in calibre-web, then synchronize them with my Kobo E-reader. Never had issues, but suddenly synchronization fails, e-reader takes long time "Checking for updates...", then finally reports "Sync Failed. Please try again."

Of course, nothing was changed on my side (...which is what users always say...)

Setup:
Linux Debian (stable) server running calibre-web from a docker container (lscr.io/linuxserver/calibre-web). Updated today, so running latest container version. Calibre-web version: 0.6.24

Basically followed the guide here:
https://jccpalmer.com/posts/setting-up-kobo-sync-with-calibre-web/
...so in basic configuration, server port = 8083, ticked 'kobo sync' and 'proxy unknown requests to Kobo Store'; "Server External Port (for port forwarded API calls)" set to 8083 as well. Note that I'm not that familiar with all this ports & proxy stuff - just following the HowTo.

How do I run the container:
TZ=$(cat /etc/timezone) sudo -u root docker run --detach -p 8083:8083 --restart unless-stopped --name=calibre-web -e PUID=1000 -e PGID=1000 -e TZ=$TZ -e DOCKER_MODS=linuxserver/mods:universal-calibre -v /opt/calibre-web/data:/config -v "Calibre_library":/books lscr.io/linuxserver/calibre-web:latest

I've attached the relevant part of calibre-web.log to this issue.
(I've anonimized the log a bit with "xxx" in some long identification strings)

Now I've googled a bit (based on log line "Failed to receive or parse response from Kobo's auth endpoint. Falling back to un-proxied mode") and found similar issues, but not sure if my issue is similar. And more difficult, not sure how to apply the solutions to my situation.

Also the line "Failed to resolve 'storeapi.kobo.com' ([Errno -3] Temporary failure in name resolution)" could point to a root cause... but, I don't see yet how.

Lastly this line in the log:
"requests.exceptions.ConnectionError: HTTPSConnectionPool(host='storeapi.kobo.com', port=443): Max retries exceeded with url: /v1/analytics/gettests (Caused by NameResolutionError("<urllib3.connection.HTTPSConnection object at 0x7f5b475cdc10>: Failed to resolve 'storeapi.kobo.com' ([Errno -3] Temporary failure in name resolution)"))"
...doing a simple test in my browser to https://storeapi.kobo.com//v1/analytics/gettests gives me a 404 error - suggesting that something is changed or wrong at kobo.com. But then I would expect many more people complaining on the internet.

So lots of information in the log, but I'm not able to connect the dots of information to a root cause. Anyone out there with ideas on how to continue debugging, or suggestions on how to solve it?

Thanks!
Matthijs

Kobo_sync_issue.txt