Originally Posted by
flyingfkb
There is always a small risk when transferring in a country where you need a visa to enter. Quite often people get stuck in the transit zone due a missconnection and can't leave the transit area to stay at a hotel outside for one night. That's the reason why some countries require a regular visa even for just transferring ( and I don't mean the Schengen transfer visa).
Anyways, Tom Tulpe explained pretty well want went wrong. Arriving and departure gate were both non-Schengen gates but are separated by the Schengen area. Probably no one at Eurowings thought of this.
Assuming EW operated both flights, “what went wrong” was EW failed in responsibility to inform authorities incoming passenger was doing TWOV.
TIMATIC indicates transfers at DUS can be made without visa as long as guidelines as followed which include same day transfer and informing DUS/border police ahead of time (presumably to escort passenger between the different non-Schengen areas).