81 comes from the "Authorized only" block page and means "Authorization failed" - exactly what you are seeing. I suspect an issue with groups, due to some authentication being broken by OWA.
I just quickly tested it with NTLM using the user DB and it works. I get all objects delivered. Are you using SSL Scanner?
ONe more thing to check. If you are using a PAC file to ditribute the load over several Gateways, then you might authenticate on one, but a different part of the page may go via a seperate Gateway.
Try pointing to one gateway and see if that works.
We had this issue and I had to change the pac file to point the URL to one pair of gateways.