Endless "Inviting"; while creating a Sandbox tester account

When I want to create a Sandbox tester for an iOS app of mine, I find myself in front of an endlessy spinning Inviting button.

Has anyone had this experience?

What are we supposed to do to solve this issue?

Since the mail address is not even supposed to be a real one. I can't really see what it could be waiting for.

Beside I have created Sandbox testers accounts before and do not remember having seen this problem.