Unable to add ESX host to AD in a nested environment

Hello.

I've just attented to VMWare 6.0 ICM official course and I'm following the official lab guide in a nested environment.

When it comes to add an ESX host to Active Directoy services, it won't work. In "Authentication Services", I click on "Join Domain", provide the credentials (user@domain) and it always gives me "Errors in Active Directory Operations" error.

All VM's firewalls are off. I am running Win Server 2012 R2 as guest operating systems.

I've tried everything. Even turning off the ESX own firewall via shell commands, with no result.

¿Any clue?

Thanks in advance.