Adding new Sandbox user fails with HTTP 500

Hi,

I'm trying to add a new Sandbox User in Itunes Connect > Users and Access. I filled all the requiered fields but when I click "invite", I get the error "An error has occurred. Try again later."

By inspecting the http request, I see that the POST to https://appstoreconnect.apple.com/iris/v1/sandboxTesters fails with

{
"errors": [{
"status": "500",
"code": "UNEXPECTED_ERROR",
"title": "An unexpected error occurred.",
"detail": "An unexpected error occurred on the server side. If this issue continues, contact us at https://developer.apple.com/contact/."
}]
}

any ideas what could be wrong?

cheers