Sign in with Facebook problem - The username is already in use.

Hello,

My site is:
Joomla 3.4.5
CB 2.0.12 Nightly Build
CB connect 7.0.0-beta.4

Facebook registration is working properly the first time I hit Sign up with Facebook. But when the user log out it is not possible to Sign in with Facebook. The system returns:
Error The username '' is already in use.
How can I fix this? I can't find the reason for this problem with joomla debug activated...

Delete the test user, enable debug mode and maximum error reporting in Joomla global configuration, then retest to see if any errors are present. It links the Joomla and Facebook user by their Facebook ID, but if that failed to save properly the relationship could be lost.

When a new registration is made I receive the following:
Notice: Undefined variable: a in /home/.../.../components/com_comprofiler/plugin/user/plug_joomdlehookscommuntybuilder/joomdlehookscb.php on line 52
When tried to login after the user is registered there is no notices but the error again:
The username is already in use.

Disable non-core CB plugins, including that one, except CB Connect and see if issue persists. Seams there's some sort of conflict present.

Thank you for your response!

First I need a way to write all the usernames in the database in lowercase. Maybe this should be done with auto actions?
I am trying to sync my joomla site with my moodle site, but moodle supports only lowercase usernames. So if I set for example [profile_firstname].[profile_lastname] for the username format I get this error:
Sign up with Facebook failed. Error: Username must be lowercase

Second, the registration with facebook is fine but the problem with the facebook login still persist. I noticed that after the registration if I go to Community Builder-Tools and click the button "Synchronize Users" then I can login with the Facebook button. It seems that the profile is not synchronized after Facebook registration. Is there any solution for that?

Also when the user is logged in there is a notification for error: You are already linked to a Facebook account.

I am trying to sync my joomla site with my moodle site, but moodle supports only lowercase usernames. So if I set for example [profile_firstname].[profile_lastname] for the username format I get this error:
Sign up with Facebook failed. Error: Username must be lowercase

Don't think you'll be able to do that given the validation is done before registration. I recommend whatever you're using to synchronize with moodle have it lowercase the username instead.

Second, the registration with facebook is fine but the problem with the facebook login still persist. I noticed that after the registration if I go to Community Builder-Tools and click the button "Synchronize Users" then I can login with the Facebook button. It seems that the profile is not synchronized after Facebook registration. Is there any solution for that?

Sounds like you've users out of sync with CB that are trying to use CB Connect. Please ensure you are only using CB and CB Connect for registrations. If you're manually creating users ensue you're only using CB > User Management.