There is one other strange thing...
When I do a
"Serial.println(ADCSRA);"
I get 135 as output. This is equal to 10000111 afaik
So: ADEN is 1 and ADPS (Prescaler) is 111 (=Factor 128)
On a reset everything should be 0 in ADCSRA.
I wonder which code is messing with this register...
Just noticed that the same output is displayed for atmega328p devices...