Hi Pepe,
some time ago when we still used MM, we did backup with the following
univention-app shell mattermost mysqldump --user=mmuser --password=THISISMYMMDBPASSWORD --databases mattermost > FILENAME.sql
In addition to the MySQL database the folders config and files should be backed up as well.
/var/lib/univention-appcenter/apps/mattermost/data/config
/var/lib/univention-appcenter/apps/mattermost/data/files
If you get an error like this
mysqldump: Error: 'Access denied; you need (at least one of) the PROCESS privilege(s) for this operation' when trying to dump tablespaces
you can try adding --no-tablespaces as option to mysqldump
Good luck!