Hi @snikch 
When I run through the login process on your site, I see this error in the javascript console. It’s coming from theme id=12, which seems to be attempting (and failing) to add some kind of extra styling for the login button?
If I use my browser’s dev tools to ‘block’ the js for that particular theme, then everything seems to work as expected.
So I think to get things working again you can either disable theme id=12, or edit it to fix/remove the problematic code.
That said, we really don’t want this kind of theme error to totally break login for sites! So I will look into whether we can improve our error handling here 
(edit: PR to improve that here)