Hello alzander,
Sorry I've not been back lately to let you know what happened. It appears to be template specific because on a clean build, other extension which use 'lightbox' or 'modal' effect are having the same problem
I have raised this as an issue on their forum, so will wait for a reply from them, and will let you know. Hopefully, they will resolve the issue (but I think they will blame 3.3.1) so I can use SCLogin because I think it's an excellent addition to any joomla site...
As Arnie said "I'll be back.....!'