I upgraded Nextcloud to the latest save without realizing the gallery app has been replaced by an half-baked “photos” app, completely useless to share pictures in any relevant way to me, in addition to a bug with ublock origin making the whole “sharing” interface disappearing.
Rolling back is not as easy at it seems: the “occ” php app check with your config version if it matches the software version, and if not matching just plainly refuses to work with:
An unhandled exception has been thrown:
OC\HintException: [0]: Downgrading is not supported and is likely to cause unpredictable issues (from 18.0.3.0 to 17.0.5.0) ()
So you need to update this config/config.php. Then, playing with occ app:list, occ app:remove and occ app:install only you can get back to a working install.
update: since then, nothing changed, nextcloud 20 has been released and it looks like the developers have no plan to deal with this issue. Since users were pissed at their refusal to consider there is an issue to fix, now they lost their interest in fixing the issue that they did not want to consider in first place.
# cd /scratch 