I just ran Autotune error check on your site and received the following error:
In order for social login to work correctly, this issue will need to be resolved. Please review our
Common Support Questions
for the "What do these Autotune Error Check errors mean?" for the initial suggestions on how to fix this.
Additionally, I just created a Joomla user on your site with the username "sourcecoast" and was able to create the user and subsequently log in just fine.