I recently rebuilt my OMV box because I was tinkering with OMV 5 installed on top of Debian (caused more issues than it solved, backed the hell out).
After rebuilding my box I could not login to the GUI no matter whether I changed the password via SSH using omv-firstaid.
Anyway it turns out the account was locked out. I suspect that Google Chrome was doing some of its usual "I'll save you time and pre-auth" garbage and inputting the old and defunct password that I used on the previous build.
In essence, when you use omv-firstaid, also take a look at option 4 - "reset failed login counter" and see if that helps.