Ok, so I ran across this message while trying to TFTP an update to a 5505 ASA.
%Error reading tftp://10.10.10.10/asa823-k8.bin (Unauthorized client.)
Now at first this was very confusing for me. I will save you guys the long and drawn out details. The punch line is that it didn't like the TFTP server I had running from What's Up Gold. I turned it off and ran the default windows TFTP service and it picked up the file just fine.
So in short, this message means your TFTP server isn't playing fair with the appliance.
Rob
