Hi Carsten, just as well your translators are more responsive
Anyway, it looks like it is a fixed rate contract and you've already released part of the payment.
It would also appear that your entire site is currently hosted on the server of the agency your freelancer is associated with?
The freelancer looks like he has a decent enough history with a 100% JSS and a long line of mostly successfully completed contracts, so there aren't any obvious warning signs there. I'd shoot a strongly worded message at him, at the end of the day he wants the rest of his money and the site doesn't look like it's too far off being done.
Also, if you go to the agency page of the agency your freelancer is associated with it says he last worked 8 days ago, maybe he is on holiday or sick or something like that?
Maybe give it another day and, if all else fails, contact Upwork to step in?