The power save modes on the Horizon are dire.
The box takes a few minutes to boot up, and then another minute or so for its internet connection to UPC to stabilise (before which the guide is painfully slow and replay doesn't work). The only solution is to either turn the box on a few minutes before you need it or set the "standby power usage" to HIGH.
Or ditch it and get a digicard and compatible TV.
Is it something like "Sorry, the box has temporarily lost internet connection" or something like that? Unfortunately, as far as the Horizon box is concerned, that is normal booting-up behaviour (except in HIGH mode, as that keeps the network connection on all the time).
I've not had to call support recently, but there used to be an English option. If not, select the German option and just ask, clearly, for someone who can speak English once they answer.
If you've already tried resetting your Horizon box back to factory defaults, then Error 2010 sounds like a cabling problem.
If checking all your own cabling doesn't solve the issue, including that thing that converts a 2 or 3-hole socket into the right connectors, then I guess UPC will send an engineer out to start checking things.