I mean the router's external IP but normally that can't be the reason, because that would mean your provider does not allow returning the packet to your IP, which would not make much sense (and as you said, my hoster upgraded something, not the whole world). But I'm still wondering how @pixelfan solved the issue by resetting his router...

The 37.17.224.60 is reachable from all over the world but not for all. I've never experienced such a strange networking issue.

I'm now in contact with my hoster... If they can't find anything by tomorrow, I will move SW to my dedicated server ;)