Joining a Samba server to an Active Directory Domain fails

# net ads join member -U Administrator
Failed to join domain: Failed to set machine spn: Constraint violation
Do you have sufficient permissions to create machine accounts?

Simple cause, found after hours and hours:

In the smb.conf were some netbios alias definitions, of which one was already defined in some other hosts SPN.

Either remove the spn from the other host, delete this (in my case) obsolete, old host or remove this alias from smb.conf.