Ah, for testing reasons I disabled allow_url_fopen in php.ini which led to not working remote (‘http://…’) schemas… 
With allow_url_fopen = On everything works fine and just the Crossref URL change has to be fixed…
Ah, for testing reasons I disabled allow_url_fopen in php.ini which led to not working remote (‘http://…’) schemas… With allow_url_fopen = On everything works fine and just the Crossref URL change has to be fixed…