Hello, to me the same thing happened. I even deleted TWINT (Postfinance) and re-installed. It seems to open, but then crashes again, but still the Intro-Screen is visible in the background if you flip through the open apps, it shows up.
I have iOS 18 Beta 4, installed on a iPhone 14 pro Max, and the latest Version of TWINT (Postfinance).
I am using the Face ID to open the app.