Finally I found out, why I can't download the Demo anymore :

In Germany there is no more access to the DEMO on the shopsite of this game due to missing age-rating !!

Please take action, rate the Demo and offer it in Germany, too (again).

Many thanks in advance - but if you need help, you might also find this explanation from another DEV, who already fixed this, useful:

https://gtm.steamproxy.vip/app/941460/discussions/0/591758483838258008/?tscn=1735549325#c591759013796953851