EDIT: Fixed in version 2.2

Thanks for the bug report. I have followed up with another bug report at https://stackapps.com/questions/8233/oauth-redirect-uri-to-https-stackexchange-com-oauth-login-successxxx-strips-p

Meanwhile, as a work-around (to get the authentication to succeed) you can open the browser console for the authentication window (F12, then press the Console tab) and run the following code: location.search = '?robw&';

There also seems to be an issue with detecting the account ID. It seems that the logged-in status is currently not displayed at stackexchange.com any more. Until that works again, you can manually find your account ID by visiting https://meta.stackexchange.com and clicking on your user profile. The ID will be the numbers that you see in the URL.