Frequently asks to "open tap", but does not work

The machine frequently asks to "open the tap" which is normally done for cleaning or pushing out air. Only in this case, once the tap is open, nothing happens. What did most of the time help is just to let it sit for a while. When it does produce coffee, presumably pressure is also lower than before, though I cannot check/compare this.

On another forum, it was suggested that this just might be calcification, but then I would at least expect the pump to try to do some work in the cases when it asks to open the tap. But it does not even try. In addition, this error also precludes proper maintenance, for example when I did the last cleaning cycle, it did not complete properly at first because it also asked to "open the tap"/did not continue during the cleaning cycle because of not pumping water.
Any ideas which component this might be related to?

Please note: "Tap" in this case might be a bad translation, not sure what the english translation used by the manufacturer in their display is in this case.