I wondered if my troubles really are sourced by the rotary encoder plugin itself, so I swiftly disabled it, happily installed Darmur’s “GPIO Buttons”-Plugin, reboot, just to be sure (yes, I’m a windows victim), and guess what?
No button click is catched, both encoders fail.
Just for fun, I tested an alps-encoder by just connecting the two switch-pins, no button click catched.