I have the exact same problem as this with a sb6120 I bought:
»SB-6120 slow to display internal status pages
I have the exact same firmware. I was able to connect to the modem to see the status page by forcing my PC netword card at 100Mbps. Seems like a faulty port or a problem with the firmware.
Anyone having the same problem ? And what happened to Ender3ed's problem? was it solved?