I have a Netvanta 6355, firmware version R11.5.0.E, that errors to a 503: server error when I navigate to Physical Interfaces on the System menu. The link works on the 2 other 6355's on our WAN running different firmware versions.
I believe this issue is fixed in firmware versions R11.4.2, .R11.5.2, and R11.6.0. All newer versions should also include that fix. Please let us know if you have this or any similar issue on the newer firmware.