fetch_rss problem, feeds docs start with 200f

I’m modifying my wordpress, but seem to run into problems with the fetch_rss function. I kind of found the cause, but no good reason for it. For some reason, when I try to retrieve some (internal) feeds the document will start with ‘200f’ and then the xml follows -> invalid, no conent retrieved.
It works fine for static feed files, and in a firefox, but the internal fetching won’t work.

Is anybody able to tell me what might be causing this?