Contact push error Joomla Plugin

Hello,

I am trying to get the Joomla Plugin to push new registered users as leads. The Joomla plugin is working, and I also set up the API connection with Mautic.
When I register a new user on the site i dont see it in Mautic. When I then check the Mautic Joomla Plugin Log I noticed the follow info and error events:

2018-02-27T17:57:53+00:00 INFO 11.22.33.44 plg_mautic onUserAfterSave method called.
2018-02-27T17:57:53+00:00 INFO 11.22.33.44 plg_mautic onUserAfterSave::isNew: true
2018-02-27T17:57:53+00:00 INFO 11.22.33.44 plg_mautic onUserAfterSave::success: true
2018-02-27T17:57:53+00:00 INFO 11.22.33.44 plg_mautic onUserAfterSave::send_registered: ‘1’
2018-02-27T17:57:53+00:00 INFO 11.22.33.44 plg_mautic onUserAfterSave: Send the user to Mautic.
2018-02-27T17:57:53+00:00 INFO 11.22.33.44 plg_mautic onUserAfterSave::mauticUser: array (
‘ipAddress’ => ‘11.22.33.44’,
‘firstname’ => ‘UserFirstname’,
‘lastname’ => ‘UserLastname’,
‘email’ => ‘userhis@email.com’,
)
2018-02-27T17:57:53+00:00 ERROR 11.22.33.44 plg_mautic onUserAfterSave::leadApi::create - response: i: i

(user info and ip is changed for this example)

Does anyone have any idea what could be wrong? Thanks in advance!