As I expected, my ISP upgraded my connection literally the day I left for 2 week vacation. While I was away, the only thing I was able to do was to update the DNS entries (my permanent IP address changed too) but there was nobody at home to reconfigure the router.

Everything should be back to normal now. I will try ADSL2+ mode tomorrow. In a strange twist of events ADSL2+ firmware for my router became available (not counting any beta releases) yesterday ;)