I've done some tests with the encoder libary on an Arduino Micro and everything worked perfectly.
However, I have now upgraded to an Arduino Due and I'm not able to get it working there.

It seems that somehow the pullup-resistors don't work. Because when the encoder is in in a state where both pins are floating, interrupts happen constantly and the value increases rapidly.
When the encoder is in a state where both pins are tied to ground, everything is fine (no unwanted interrupts).

I've also tried the following code and it works as expected:
void setup() {
	pinMode(12, INPUT_PULLUP);
	attachInterrupt(12, testfunc, CHANGE);
The function "testfunc" is called when the pin is connected to ground and when floating there is a constant 3.3 volts.

After initializing an encoder at two pins, their voltage is somewhere between 0 and 3.3 Volts when floating.

Does somebody have an idea what's causing that and how it can be fixed?