How to activate external beta testing in ITunes Connect for a new version after previous beta review approved?

I submitted a new build from Xcode.
I have a pre-released build on Itunes Connect.
I filled in all information (by clicking on the build version) about this pre-version.
I added external users (They have "Added" status)

But when I turn on beta test flights, external users stay at inactive status ! For a previous version, it proposed me a "send for app review" instead which was successful, but now I cannot use external test flight ... I already read the apple documentation several times ... Is somebody also facing this issue ? Did I miss something ? Thanks for your help !

Solution

Apple would appear to have fixed the issue. I was facing the same issue but this morning I can submit the new versions for external testing review.