Uh, his is a good point.. and a stupid error due to first time programming for HTTP communication!
I was sure that since I was pushing some variables, the server did not have to answer.. instead the GET method requires an answer in any case.
So, adding the reading of the answer is better.. fewer or no errors.
Sometimes (some hours) it still stops working, but I think it is related to some out-of-memory issue (my real code is about 20K.. there are a lot of debugging checks and Serial.prints). I'll try to reduce it and to increase the delay between two server requests and see if the code remains stable for some days..
Thank you Surfer and other people, it was an honor to have prompt answers from all of you!
(.. and Happy New Year)