This issue occurs due to the presence of Cisco bug ID CSCei08457.
This problem occurs on CiscoSecure ACS version 3.3.1, which performs Protected Extensible Authentication Protocol-Microsoft Challenge Authentication Protocol version 2 (PEAP-MSCHAPv2) machine authentication to the Active Directory (AD).
Machine authentication fails, and the AD displays the error 1326L message if the machine name, excluding the domain part, is more than 15 characters.
As a workaround, ensure that machine names are always shorter than 15 characters.
The fix for this bug is available in CiscoSecure ACS for Windows version 4.0(1.27).