A good post about Kerberos Authentication problems with mixed 2003 and 2012R2 Domain Controllers – Ask DS Team

The Kerberos client depends on a “salt” from the KDC in order to create the AES keys on the client side. These AES keys are used to hash the password that the user enters on the client, and protect it in transit over the wire so that it can’t be intercepted and decrypted. The “salt” refers to information that is fed into the algorithm used to generate the keys, so that the KDC is able to verify the password hash and issue tickets to the user.

When a Windows 2012 R2 DC is promoted in an environment where Windows 2003 DCs are present, there is a mismatch in the encryption types that are supported on the KDCs and used for salting. Windows Server 2003 DCs do not support AES and Windows Server 2012 R2 DCs don’t support DES for salting.