We're running Resolve 15.0.0B.073 on Win7Pro. When we attempted to reinstall in order to change the PostgreSQL option, as advised in the Resolve Project Server error message box, the installation failed with a warning that there is currently a newer version installed. As Resolve 15.0.0B.073 is the same version, this error appears a little bogus.
Resolve 15 and the Project server is designed for W10 and so any issues you find W7 could easily be OS related. Beta8 was just released so you could try that, but hopefully you backed up your DB first.
Uninstall the current version first -- "bogus" or not, you can only install a later version over an existing version. Same or earlier versions will generate the error message you saw.
Uninstalling through Windows won't remove Resolve defaults and local databases, though I can't speak to PostgreSQL, no experience there.