I've been frantically busy for a few weeks, but I did get a few hours to investigating this yesterday. I think I have identified the error but have not yet got a solution to fix it, as all my attempts so far have broken the server

I believe it is down to the content size header bearing the incorrect size for partial content responses, but the response handling code is so monolithic that every attempt to change it breaks something else...
With a bit of luck, I'll have something for people to try soon.