It seems the problem is solved!!!! (I hope permanently). Thanks to Don, again!
The terminal in the M2 controller output and the terminal of the cable to the motor were very dirty, because of the melting that I mentioned (mainly melted plastic). So I cleaned them as much as I could, replaced the bolt and washers and put all back together. I had a short ride today and so far, so good, any throttle failure at all. In the past, this ride length was more than enough to cause the problem.
I checked the controller top when I came back home and everything seemed ok. The M2 output was not hotter tan M1 or M3. Anyway, I’m going to look for a thermometer that can measure it and check it more specifically.
My hypothesis are (based on what Don said):
The bolt of the M2 output was loose, so the contact between it and the cable terminal was not optimal. The less surface of contact led to a rise in temperature, producing melting of metal from the bolt washers and plastic from the controller top. That led to less surface of contact and increased temperature and so on…
Since the controller and the motor temperatures are ok, there was no error code in the dashboard (once again, I hope).
Any ideas of the things mentioned above of about precautions to follow from this point would be appreciated
Long life to electric bikes!