Logging in to the Bookmarklet takes me back to the login screen but no error message. How come?
The bookmarklet uses third-party cookies to check your CrossBrowserTesting.com login via the page you are testing.
Check your browser's security settings for allowing third-party cookies. You can either turn this on, or if you purposely have this off, add CrossBrowserTesting.com to your browser's whitelist / trusted sites.