Dimitris,
We've been trying to recreate this issue, but can't. We've also never heard of it before, which means that it's likely something more specific to your site.
Can you please try recreating the problem on our demo site? The SCLogin module can be found on the page below. First, click the Register button and create a normal Joomla account. Then, activate it and login. Then, click the "Connect" button and see if that works:
demo.sourcecoast.com/index.php?option=co...cle&id=11&Itemid=175
Depending on how that goes, it will help us understand how to proceed. The only reasons I can see issues happening on your site is if you have browser caching or some other agressive caching enabled.
Let us know how that goes and we'll keep trying to understand why it would be happening on your site. If we can't recreate, we may need admin credentials for your site to understand your configuration more.
Thanks,
Alex