Somebody suggested that you may be able to fix the restored install by forcing a self-upgrade (ie staying on the same version but regenerating config etc):
yunohost app upgrade nextcloud --force
Then regarding the failed upgrade attempt:
Error: App "Video converter" cannot be installed because the following dependencies are not fulfilled: Server version 27 or lower is required.
This is the classic issue where one of your nextcloud app is not yet compatible with nextcloud 28