I don't know what browser you're using, but can you try to open the Developer Tools.In the "Network" tab, hit "Preserve Log". Then try to login.
If it fails, check the Network tab and see if any requests are marked with 400 (or higher) error codes. If so, let us know what those URLs are.
One other thing to check is in the Global Configuration area of Joomla. Set the Error Reporting setting to 'None'. While I don't know of any PHP warnings or errors our check causes, if some other extension is causing a PHP message to be thrown, that could prevent our code from executing properly.
I hope that helps, but if not, please let us know.