The VCard Parsing error was a red herring, it was due to a Webcal subscription to a deleted calendar.
Tried the update again and ran into the same WebUI and desktop client throwing 400 bad request errors.
Thanks for this really nice tutorial.
My main problem was only that I “lost” a few php modules (because I was not so clever installing them on 7.4 and not the generic ones), but after reinstalling them (now generic and not just 8.2), all went very smoothly and I went directly to 27 
1 Like
Thank you so much!
It is very important that, before you do anything, you update NC to the latest release. This means that at this moment, before you start updating Debian you need to update NC to 25.0.13. Otherwise you get stuck because the updater only updates from, lets say 25.0.8 to 25.0.13 and then not further. It does not offer NC26. No clue why. 
You can run the updater as many times you like, it only offers 25.0.13 and not 26.x.x
I had now two instances with this “bug”.
The only way to fix this seams a manual update. But this is a lot of work.
Right, that makes sense. It is normal that you must update to the latest patch subversion before a new major version is offered. But just in case you forgot about that, you can also just re-apply the patch after having upgraded to latest NC25 to continue the upgrade to NC26:
sed -i 's/>= 80200/>= 80300/' /var/www/nextcloud/lib/versioncheck.php
This turned out to be a much better way to upgrade than the method in Upgrade manually — Nextcloud latest Administration Manual latest documentation, which was a rolling disaster. I had to run updater.phar several times, but each run was successful and upgraded the system, unlike the manual’s procedure. Thank you.
1 Like
To be fair, the CLI updater is as well part of the documentation: Upgrade via built-in updater — Nextcloud latest Administration Manual latest documentation
But the challenge to break the Bullseye/PHP 7.4/NC25-26 barrier indeed is missing. I am glad that this HowTo helped 
.
1 Like