To clarify, this occurs when you are attempting to log in with Joomla credentials, not social login, correct?
* One of the most common causes of this issue is related to caching on your site. This involves disabling more caching settings than just the one in the SCLogin module. Please disable caching in the Global Configuration area, the system cache plugin and any third party caching plugins and re-try to reproduce the behavior. I'd also suggest turning off the optimization on your site for JCHOptimize while trying to narrow down the problem.
* You should also try to recreate the behavior with the standard Joomla login module. This can help rule out SCLogin as the culprit.
Let me know how the above affects the behavior and we can go from there.
-Melissa