@PeDre & 2N3055,
Yes, you are right! I already had the feeling I was overlooking something. Taking another look at the screenshots, I indeed see that when the digital channel is turned on, the scope greys out the "High Res." indication on the top line. (It did not jump back, however, to sample mode, which explains that when I again disabled the digital channel, the scope went back to HighRes.)
Thanks for the swift replies!
@Tautech: Yes, should have returned the device to its default settings (called 'Preset' on this device). Just returned from a double intercontinental trip, must have been tired. Next time ;-)