It was Office 2013. I brought the PC to a shop to have it uninstalled. I tried already a device from Microsoft, I have deinstalled it, and now I get an error if I try to reinstall it from my Microsoft Account. I have given up :-)
Will keep you informed, thank you.
After switching on and off 3-4 times, now the watermak is disappeared. Now the problem seems to be Office: it does not work properly, it seems installed 2 times... I was able to deinstall one of the instances of Office, not the other one :-(
The solution explained in the first part of this discussion does not work for me. I have tried 3 o 4 times. The watermark says: "Test Mode Windows 8.1 Build 9600". I DO NOT want just to delete the watermark, I do want to get out from the text mode. Apparently I am in a test mode after an HP...