I only know of 'The boy trapped in the matrix'

Do you mean specific cases, or just random folklore? For the second i know of the "wechselbälger" which is basically that one night your little child gets kidnapped, and exchanged for one that looks just like it but isnt

Random folklore. I'm creating a worldbuilding project set in a post-Revelations Germany.

>>17903099
The are my family comes from has severla such stories. Mentioned Wechselbälger for instance. If you disturb the "Unnererdschen" they might steal your baby and replace it with one of their own. This was probably used to explain mental and physical deficits or just ugly children. The Unnererdschen ("below grounders", gnomish fellas) live underground but have their certain entrance places, that could be a hill or bush. To keep them happy, just leave some bread or cake for the from time to time. People know where to find them.
Witches are a fact, you cannot leave gardening tools or cattle outside in the Walpurgisnacht (night before 1st May).
There are witchers, too, called "Brukers". Generally they heal people by driving out bad spirits that cause illnesses. He finds out which Spirits are involved by throwing different sorts of Wood - each representing one spirit- into a water basin. Depending on which sink, he goes to work.
Then there is some sort of Vampire, the "Duwwelsügger" (double sucker) who drains life energy from people. He doesn't leave his grave though, so it's a pain to find the culprit. Since his body doesn't rot, it's easy to see who's a duwwelsügger though. Placing a copper penny in his mouth stops his doing. Should've done that before the burial.