I need to start AR - go back - start again so that the ar mode works

I’m starting to develop AR projects in three.js, testing the demos on my server and making minor modifications.
But I always run into a problem, even in the original demos: the “Start AR” button is not active, and if I press it the screen goes black. But if I enter AR, go back and enter again, it works fine. Where is the problem?

I had the same problem. Additionally, it worked on ios when using an app XRViewer, but not on android. What solved it for me was updating the threejs library, since I haven’t updated it in a last year.