@iaresean try to check if the related application pool on IIS7 is running in "integrated" (not in "classic") mode.
The issue seem to be hard to reproduce locally, if you can send a copy of your website to ddz (at) composite.net, we will try to debug it next week
The issue seem to be hard to reproduce locally, if you can send a copy of your website to ddz (at) composite.net, we will try to debug it next week