Addendum: After the support wrote me 2 weeks ago that the Opener is connected in the same way as the HT3009/2 ALPHA, I have now been using the Opener for 1.5 weeks, and so far no problems at all.
Thanks, that did help!
Apparently there are at least two ways how the HT2009/2 Alpha can be wired to the door. One being your setup with a separate cable for the ringer and in that case you follow the Default instructions from Nuki. The other case is a more bare Setup with only Ports I, II and E connected, without a separate line for the ringer. In that case you have to follow the instructions specified for that case (Leaving out the Yellow cable and the clamp, Connect blue, Green, black to I, Connect red, orange, Purple to II, in addition to the existing cables)