Thank you Reggie,

My DNS also was pointing to OpenDNS. I've realized that my configuration is not well supported and the cause of the problem could be the Zyxel router . Now, I have finally been successful in setting up the Zyxel in bridge mode (quite tricky...) and the AE in PPPoE mode (very easy)... everything is very smooth and generally better. Most important, the problem is apparently gone!