wolfgang, i have test that in my Quad, it all works. and when input is 2.6v the vpp and vdc are all about 2.58~2.65.
i think it maybe caused by calibration data is not stored eventhough you select the “exit and save calibration”, would you help to test: when in calibration mode as you calibrate for the 0.5v , adjust the data to 2V when the input was 2.6v, and save it .reset the Quad , and input 2.6v voltage and read the wave on the screen . to test if the Quad stored the calibration result .