There should be a switcher next to the date field, allowing you to input any text in it. Furthermore, if you start typing '{{' into the field, it will automatically switch to the "any text" input mode. If you don't see it, can you provide a screenshot? If you do see it now, can you provide any suggestions to make it more obvious?

I'm a bit confused. Now the form works as you say.

However, before ca. half an hour, I didn's see the switch and when I tried to copy the complete "other date" template to the field, the field get red and the form refused to accept the entry. Either i had a hallucination, or...

Or you have the UW page open in a tab all the time and don't refresh it for days? Some people seem to do that. I wonder if some kind of auto-update feature made sense.

This was changed very recently, see T110026, because although the switcher was always there in the new design, it was hard to notice. It used to look like this:

In fact it seems like the change was deployed less than two hours ago, so you very well might have had an older tab open. I'm glad it works now; I think we can close this task? Thanks you for diligently reporting the isue :)

I have surely not the UW page open for a time, I use it very rarely (I prefer Special:Upload for my own photos).

Let's suppose that I had an eclipse, I overlooked the switch and maybe the template was copied with initial space character?...

I think, the task can be closed as mistaken or solved.

For possible investigation: the file i had problem with was https://commons.wikimedia.org/w/index.php?title=File:Star%C3%A1_Balabenka.jpg&oldid=170494417