I have run across this in the past where a 5330/5340 phone sets up remotely and downloads the firmware and accepts the extension info, but whenever it attempts to download the HTML info, a default screen saver error pops up. I discovered that some feature functions such as music worked, as well as the phone ringing when dialed, so it seemed it was actually an HTML issue being blocked somehow.
It seemed this was a firewall error and even though the network admin opened up the ports there was still some faulty firewall settings on the phone end. When I was able to set one of the remote phones up on a public IP it worked flawlessly, but when it was NAT behind a firewall, the same issue appeared. There ended up being a change in service providers and equipment at the remote end which solved the issue.
I know this doesn't solve your problem, but it may arm you with some info to use against your service or network provider.
Jim