I can log in to the WebGUI, but the modem reverts back to the login page after 3-4 seconds, requesting the password again.
This behavior originated from downgrading the firmware from 6.07.5 to 6.06 and the modem not accepting a new password.
I downgraded the firmware to something around 3.906 (back then passwords didn't need to be changed) and then back up to 6.07.0 and it once accepted a new password and this works now (otherwise I get the Invlaid Username/Password message), but the WebGUI is not usable in this state.