I’m not familiar with OMV myself but if it uses standard deb installs then that might apply to your install:
For specifics beyond that you’d have to check with somebody more familiar with the Linux side of things.
As for the jobs, I don’t know if an uninstall will delete the job files or not - again, it may be an OS specific “standard” of which in not aware.
As for the GUI password, could that have been cached/stored in your browser?
Note that @Watson’s question is valid, did you reinstall the same version or switch to a newer one than you had?