When you install or update Azure AD Connect, the following error appears:

Unable to validate credentials. An unexpected error has occurred. The underlying connection was closed: could not establish trust relationship for the SSL/TLS secure channel

The sources of this problem may be (non-exhaustive list):

• proxy
• firewall
• TLS version used

To detect the root cause, you can use the excellent script Azure AD Connect Networkof Aaron Guilmette from Microsoft (his blog).

On my side, the problem was a proxy issue on https://login.windows.net:

Lors de l’installation ou mise à jour d’Azure AD Connect, l’erreur suivante apparaît :

Impossible de valider les informations d’identifications. Une erreur inattendue est survenue. The underlying connection was closed: could not establish trust relationship for the SSL/TLS secure channel.

Les sources de ce problème peuvent être (liste non exhaustive) :

• blocage proxy
• pare-feu
• version TLS utilisée

Pour détecter la source de l’erreur, vous pouvez utiliser l’excellent script Azure AD Connect Networkde Aaron Guilmette de chez Microsoft (son blog).