I updated from Nextcloud 13 to 14 yesterday. I tried updating via GUI as usual but I get an error and then maintenance mode stays active. I tried updating via CLI and same problem. Please see screenshots for further clarification. Any help regarding this would be highly appreciated.
you should try a manual upgrade (i think updating with gui is not supported for this version yet).
stop webserver
rename your nextcloud-dir.
download 14.0.3-release and unpack it; put it in the same path/name as the old nextcloud-dir prior to renaming.
copy the config.php from old dir to new dir.
3.5 if the data-dir is inside the nextcloud-dir copy it also from old to new
adjust permissions of new dir as necessary
run occ upgrade in new dir
done (you might have to manually turn off maintenance mode)
downgrading is not officially recommended but possible if you have a recent database-backup (dump) for the old version (your database has already been altered by the gui-updater).