Martin, Rachael,
I wonder whether you are running IIS max path limit issue. To check this try out
CustomDownloadURL instead of DownloadURL and set bindsession to false and/or iTimeOut to 0 as they both increase the URL length.
If so read
https://stackoverflow.com/questions/...ist-in-windows
AFAIK the web.config setting to increase the length does not work but there is/was a registry entry for it. I however don't know anymore what this entry was.