Skipped major release - now: PHP Fatal error

I was running nextcloud on a synology using docker and had the idea to change the tag from stable-apache to apache, so I can have the newest version (27) and not the stable one (25). Well, I didn’t inform myself beforehand and skipped the version 26 and a lot of problems happened from there on. I followed a github issue and post here to fix it, but I somehow couldn’t solve the missmathed apps version problem. So I went manually and changed all the min-version="27"values to min-version="25" (with grep -l -r 'min-version="27"' | xargs sudo sed -i 's/min-version="27"/min-version="25"/g'). The update did succeed in the end, but now I am not able to run any php occ command at all, giving me an error:

PHP Fatal error:  Declaration of OCA\Deck\Db\RelationalEntity::markFieldUpdated($attribute) must be compatible with OCP\AppFramework\Db\Entity::markFieldUpdated(string $attribute): void in /var/www/html/custom_apps/deck/lib/Db/RelationalEntity.php on line 56

I wasn’t able to find any similar issue, so I wanted to ask how I could fix this problem…
The error comes up every time I run the occ command, so that one is out of question.

That error is coming from your Deck app. I suppose you could disable it.

Though what I would do if I were you is return to a backup from just prior to getting into this mess. Then upgrade cleanly from NC25. Even if you get it to “work” there is no telling what weirdness is left that will bite you during some future upgrade or use case.