5.7.1. Connecting to Marketplace causes fallback error

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If I try to connect to the marketplace, the page "http://www.christianmichael.eu/index.php/dashboard/extend/connect/register_step1" opens for 2-3 seconds and then it falls-back to the homepage, showing following link: "http://www.christianmichael.eu/#14138727277281&0"

Tried to stop loading during process with stop button of the browser, then the site stays.

So I filled in the connection form and hit the button connect to community, then it falls-back again to "http://www.christianmichael.eu/#14138727277281&0" and nothing happens.

Could somebody please give me a hint?!

Thanks!

Lupo777