Thank you @johnwasser and @jhaine, that was helpful and clarified a lot.
Obviously I didn't "get" the sinewave offset, even though everything was positive from GND.
To try to circumvent the offset issue for a moment, I used my AWG to output a 3 Vp double rectified sinewave. This is also the intended input for the system this test is part of.
My sketch and the DSO again agree with the (calculated) RMS values for this waveform (2.08 & 2.1Vrms), but the DMM's are not in agreement with 0.916VAC, so the original question was still open for me.
The comments about the offset put me on the right track however, so this is also for others following this topic.
The reason why the DSO RMS calculation for the waveform are so different from a true RMS DMM is that the DMM's measure the AC coupled waveform. When I AC couple the DSO input, the RMS voltages agree.
There are several examples for RMS calculations and scripts out there, similar to what that I use, so be aware.
I have seen libraries that take the offset into account. I need to examine them.