Error on Login or Register

Hey guys,

the plugin worked perfectly up to this point. When I test to login with facebook now, I do get an error page with the message:

array(1) { ["{"access_token":"***here stands the access-token which i removed for the forum***","token_type":"bearer","expires_in":5183932}"]=> string(0) "" }
Something went wrong 

What is really weird, is that if I try it in a Incognito tab or on a device which has not yet been registered via the Facebook login, everything works fine. So the problem only appears on devices where someone has once logged in via the Facebook login, has then logged out, and then tries to login or register via Facebook.

If you need any further informations, please let me know! Any help is really appreciated! Thanks guys!