One more word: there was a warning section in result that it has been truncated. I don't really believe this is connected to the problem, but also can't exclude it at the moment.
How do you see that?
Is it related to this? https://bugzilla.wikimedia.org/show_bug.cgi?id=26339
If so, is there a way to iterate (gently) past $wgAPIMaxResultSize
jrf