Recently Google has made changes within in their authentication process that have left desktop apps like ours unable to authenticate user accounts.
Unfortunately, Google has not provided much information about what has changed and is not close to resolving this. In the mean time you can still sign in through the Plays website, go to connected accounts, and unbind the Google account. Unbinding the account will prompt you for a password. Once this is finished, you will be able to sign into the desktop app using your username and password.