social network registration error

I am trying to use the social network feature of the plugin but none of the google/facebook registration seems to be working.

for google it fails right after clicking on the link ( which redirect the page to mydomainname.com/?uultrasocialsignup=google and it shows (Server error 500) )
for facebook it redirect to facebook website and after getting the permission it fails to register

I would really appreciate if you could help me with this matter.

Thanks

https://wordpress.org/plugins/users-ultra/