I came just back to update my post, when I saw your comment.
"Nothing happens without reason" -> true, but sometimes the errors are so hidden and are random that who knows... ;)
I wanted to update that other scripts on that server are having also some random mysql connection errors. I have made some changes in mysql conf file instead listening from "localhost" to "server ip" (found this in google) ... and it seems to work now.
Because it was so a random error, that at the beginning only appeared in a recent upgraded ezp 3.10 install, I had to ask here...
Now we can be sure it hasn't nothing to do with the upgrade or ezp ;)
