So, you can not even see the media file names? Try creating a sub-folder in the shared one and see if you can see it from boxee. I don't know what boxee allows you to see or how, but I for instance can see the media files and can navigate the folder structure but if I try to play one of the media files it will almost freeze my TViX waiting for the stream to build the buffer, that slow it is.
Is boxee extracting any information from the media files during navigation of the file/folder structure? Maybe it is trying to read something, times out because of the low speed and then show the error?