Built in gallery won't update

Whenever we’re changing a native wp gallery on http://www.horizonte.com/en-german-courses/community-photos/photo-gallery/regensburg
it doesnt’t update.

Text content in the backend is
[gallery link="file" ids="6036,6769,6021,6348,6869,6754"]
and it displays the correct pictures in backend.

In frontend, the gallery simply won’t update…no cache plugin installed. Any idea?