Halloween and its custom of going trick-or-treat came to Germany in the late 90s. Since then, among some of the expat children out for trick-or-treat, there had been some unpleasant experiences. Not all Germans know or recognize Halloween, and if you do ring a stranger’s house, he might chase you away and then you are the one who’s scared. Angry German can sound pretty scary.
I have been asked “How do I know it is safe and OK to ring the doorbell?”. It is safe and OK, if you see a Jack O’Lantern in front of the house. This is the sign you are welcome to ask for treats.
This information was given to me by a German mom for the Oberursel area.
Of course, I would always advise to go only to friends’ homes or other expats’ homes. Living the international school life, they are familiar with the custom.