@Signor Amministratore @Michael Vogel for future reference, the problem was caused by a nginx reverse proxy in front of the apache server.. with these headers it seems to work again, but I am not sure about it..
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-Host $host;
proxy_set_header X-Forwarded-Port $server_port;
proxy_set_header Host $host;
proxy_set_header Proxy '';