Hi cassianotartari and highmaan81,
Hereby an updated status. Unfortunately, it is not very useful for you.
The phone was sent to the shop where I bought it. In their RMA terms, they state that you should only send the phone, cover and battery, no SIM-card, SD-card, cover, charger etc.
After they had confirmed that it had not been misused, rooted, in contact with water etc. etc., they sent it of to the nearest HTC repair shop. I am based in Denmark (and so is the shop) and I think it was shipped to Germany (based on the shipping labels when it was returned). This only took a couple of days and I was informed that the repair normally takes 1-3 weeks.
After exactly 3 weeks in the hands of HTC, I got an email that it had been repaired and was shipped to me. A few days later I eagerly opened the parcel to see the content. I could immidiately see that the screen had been replaced (I had a few scratches and a dust-leak between the digitiser and the screen). The cover (with a name tag on the inside) and battery were the original. A closer look revealed that the complete "phone"-part must have been replaced (or ridicilously thoroughly cleaned). Serial no., IMEI no. etc. is the same as on the old phone. The delivery note did not contain any information about what they had done to the phone.
As you can see, I am not able to point you in any useful direction in regards to where or what to target first in your problem solving. As none of you have any warranty, the best bet on where to start must be a balance between:
1) Probability of being the root cause
2) Price for spare parts
3) Risk of damaging even more during the repair
Both of you have mentioned a flexible cable as a possible place to start and I think this scores high on both 1) and 2). However be aware of 3)!
The way this fail has developed for the three of us, it could very well be a cable error or a problem with the connector for the cable. I have repaired quite a few flexble cables on different Nintendo DS Light, as the cable to the upper screen often becomes defective. These cables are quite delicate to handle, but I find the real challenge being the connectors - both avoiding breaking them when opening them and disconnecting the old cable and also getting the new cable connected properly.
Good luck on solving your problems. I hope you will keep this thread alive for sharing your experiences.
BR and a happy new year,
Dan