Bob thank you very much for this advise. I finally tried what you suggested to drill it successively with larger sizes. I was prepared to completely ruining the thread. I tried one smaller size drill bit and drilled all the way through, then inserted a replacement screw, it actually went into the thread and turned one revolution! So I tried a bigger drill bit and drilled again, slowly and steadily, and afterwards I tried the screw again and it went in, with slight resistance but the threads are fine! I have to pick out the pile of metal collected at the bottom of the hole but the original threads are still good. Thanks again!
MC