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Now when you want to register on Frontend if you click on the second page on "Registrieren" the page just reload and don't go to next step. See:
screencast.com/t/HknKCztZ9x
Please check again I just fix this, this issue caused by this file components/com_community/templates/default/register.profile.php, i changed the form action to be like this :
What did you change? seems you hack the code right, or install the 3rd party extension, because after register it will redirect to com_jmarket and looping error before go to com_jmarket.
If you wanna hack this, please do it carefully, please test Jomsocial in clean Joomla installation, make sure jomsocial is working fine there first.