after some more experiments it looks like I was caught by a looking at cached data. I was reloading the file for https as it was displayed as text (happens to be readable xml). However this reload was answered with not-modified without content-type (which is afaik standard conformant).
Only on hard reload Chrome got a new copy with content type header.
Goofish or not, maybe others stumble accross the same problem and this will help them to resolve the issue.