What are you using for the IP range on your LAN? (drop to a command promt, type "ipconfig"(no quotes), tell us what you get for ip address, subnet mask and default gateway.
When you request the modem's diagnostics/config page, the bridge sniffs it's own address and rather than passing them on to the UBR it responds itself.
So for some reason i think your likely to have an IP range conflict, the cable modem doesn't seem to be receiving the request for
http://192.168.100.1
..................Standing by to be corrected by Broadbandings though!