first of all: I solved my problem - I just wanted to share it.
After problems with my old setup, I decided to start from scratch - a good opportunity to clean up some stuff.
Then something really spooky happened: Alexa or Echo all of a sudden showed devices that I hadn’t set up as items yet. I did a lot of reading - of course they interfered with the ‘new’ ones. After hours of reading (to no avail), I found the reason for this mystery: I had opened some of my old files in VS - and the items got exposed to the openHAB system!
As soon as I closed the files - tata - the mystery items were gone!