Are you using http or https? Have you checked the OpenWebif settings from the plugins menu?
When I updated my Vu+ Ultimo from Apollo 100 to 106, the online update turned https off in the OpenWebif settings.
I started the update through an OpenWebif https session, and wondered why I also still got the error message of unavailable page about 15 minutes after starting the update, when the box should have finished rebooting.
I went to the living room and checked that indeed the reboot was ready, but when I went to the plugins menu and checked the OpenWebif settings, I noticed that https had been disabled in the update.
When I turned it back on, the refresh button in the browser worked.
This looks like a bug in the update process. I haven't so far noticed other changed settings and don't know whether it happens with other receivers beside ultimo or updates from other images beside Apollo 100.