the library wouldn't load. using the build from issue 3650 fixed it. but then I couldn't sign in. a totally separate issue, resolved here (issue 3545) by installing the latest release from this handy webpage. everything was working. then I clicked the "try YMP (beta)" and it fails, and I can't revert, even after deleting everything and replacing the files. how the fuck? turns out there's some .config file i can delete to reset it as per my frustration in PR 3647.
solution is deleting the whole Google Play Music Desktop Player folder under .config did the trick. loads up like a fresh install.