I still don’t have a solution, but maybe some information:
I testet the address in the screenshot of Couchabenteurer yesterday & it worked up until now- today not anymore.
I already mentioned that I have the same problem with my websites.
My url’s are:
http://www.xonk.de
http://blog.skythief.de
http://aachen.pubhd.com/
When I tested the mentioned url’s in this forum using utrace.de I detected that all url’s seem to be hosted at strato, similar to mine.
Another interesting point for me is that one of my url’s has an ssl certificate. When I test the domain using the https instead of http I get a different error. In one case of my subsites the TwitterCard even works.
This subsite has a playercard and makes it therefore an exception:
https://www.xonk.de/biologie/zoologie-evodevo/themen/das-cardiovaskulaere-system-der-vertebraten-am-beispiel-des-zebrafisches/
All the other subsites tested under https show:
INFO: Page fetched successfully
WARN: No metatags found
This isn’t a solution, but it indicates that it might be kind of a port issue between strato and twitter.
Maybe a filter at strato blocks a specific port or the IP of twitter. Otherwise it could also be that twitter blocks strato.