No, I overlooked that. That might have helped disable maintenance mode, but meanwhile, I managed to repair my problem as such on the console:
sudo yunohost app shell nextcloud
(enter password)
php occ app:disable uppush
php occ maintenance:repair
php occ maintenance:mode --off
After that, Nextcloud was running again. I could reactivate uppush.
Deactivating uppush was just an attempt because in a previous issue that app caused a similar problem (see Internal server error upgrading Nextcloud / UnifiedPush provider server app - #7 by matalb).
Thank you for your help.