listbox losing multiple selections thru proxy, but not locally

[bthomasIDFG]

Multiple selections to listboxes (but not checkbox lists) are cleared on
postback when connecting through a proxy on the internet, but not when
connecting directly to the server on the intranet or on my development
machine. Single selections to the listbox works. This is not hosted in a
web farm type setup. SelectionMode="Multiple". Again this works locally.

See an example at
http://fishandgame.idaho.gov/ifwis/huntplanner/listboxsilliness.aspx
Make a multiple selection and press the submit to postback. You will see
viewstate is preserved in the checkboxlist and radiobuttonlist but not in the
listbox. If only one item is selected in the listbox state is preserved.

What is going on here? Out of ideas (for the day at least)