I've successfuly gotten file transfers working with group chat, from the Vista x64 sp2 box, to the Server 2008 x64 box..
But, if i go into the chat window and try to click on the file i uploaded, i get an error as above.. if i click continue the error box goes away, if you hit quit it kills chat.
Also.. i cant seem to upload anything greater than 10mB.. is this an IIS setting or a group chat setting somewhere?