I have shared a file with an external user. The system sent the invitation mail. The external user has filled in the first login screen. The you should get a second mail to enable the account. But the external user has never received this second mail. Now he cannot login. User name and password are correct but the login mask rejects the user "Sign in failed. Please try again"

How it is possible to resend the second confirmation mail or how it is possible to enable the user. (in the admin interface the user is visible and enabled)