To make a long story short (like you did in the OP), I have to same problem

The cause: flashing newer firmware version. All the symptoms are exactly the same. When I get home from work today I will try these 2 things, they are a little variation of what I already tried:

http://htc-one.wonderhowto.com/how-t...a-adb-0154361/

http://www.androidlegend.com/how-to-...ndroid-device/

I don't have much hope that I can fix it myself and am already in contact with HTC though. The main problem is that I'm in Argentina and I bought the phone in Germany..

You didn't find any other possible fixes, right?