Azure AD connection via Azure AD Connect does not work

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 Network of Aaron Guilmette from Microsoft (his blog).

On my side, the problem was a proxy issue on

After the proxy was reconfigured, the connection from Azure AD Connect to Azure AD is OK.