I set up a fresh OMV4 vm for testing, patched it, included OMV Extras and installed nginx, letsencrypt and mySQL.
After starting mySQL, I tried to log into the management tool as root. I tried no password, my root password and I used the function to specify a new root password, neither of those options worked for me to login.
I want to setup a database for nextcloud.
It probably would be better to use Docker, but I am a little confused using this nice tool. I did not get my nextcloud running within docker.
Edit: I tried to upgrade directly from OMV3 -> OMV4 with running plugins (Letsencrypt, MySQL, Nginx) and migration worked quite will despite some PHP issues for Nextcloud (GD, ZIP, cURL missing). Also I was able to login to my mySQL-install using the provided web tool. On a fresh OMV4 I still can't set my root password for mySQL through the OMV GUI.