Hi Joost
Have you been able to get more clarity on this issue? I've got the same problem and although the guys have submitted this to the Google forums, there are 0 replies from Google on this. (http://productforums.google.com/forum/#!topic/webmasters/9uEIxf4FuVI)
This seems to be widespread and I'm hoping there is some way around this issue. Would it be better to submit the sitemaps individually rather than the index? I would really appreciate your feedback on this.
Thanks,
Justin.