Event 4625 Audit Failure NULL SID failed network logons

On a few of our Windows Small Business Server 2011 or Windows Server 2012 R2 Essentials servers of relatively limited use (domain controller / Active Directory and DNS, Group Policy, DHCP, file server, print server, and third-party backup) ~30 to ~1,000 of the following events are logged daily:

An account failed to log on.
    Subject:
    	Security ID:		SYSTEM
    	Account Name:		SERVERNAME$
    	Account Domain:		DOMAINNAME
    	Logon ID:		0x3e7
    Logon Type:			3
    Account For Which Logon Failed:
    	Security ID:		NULL SID
    	Account Name:		
    	Account Domain:		
    Failure Information:
    	Failure Reason:		Unknown user name or bad password.
    	Status:			0xc000006d
    	Sub Status:		0xc0000064
    Process Information:
    	Caller Process ID:	0x1ec
    	Caller Process Name:	C:\Windows\System32\lsass.exe
    Network Information:
    	Workstation Name:	SERVERNAME
    	Source Network Address:	-
    	Source Port:		-
    Detailed Authentication Information:
    	Logon Process:		Schannel
    	Authentication Package:	Kerberos
    	Transited Services:	-
    	Package Name (NTLM only):	-
    	Key Length:		0
    This event is generated when a logon request fails. It is generated on the computer where access was attempted.
    The Subject fields indicate the account on the local system which requested the logon. This is most commonly a service such as the Server service, or a local process such as Winlogon.exe or Services.exe.
    The Logon Type field indicates the kind of logon that was requested. The most common types are 2 (interactive) and 3 (network).
    The Process Information fields indicate which account and process on the system requested the logon.
    The Network Information fields indicate where a remote logon request originated. Workstation name is not always available and may be left blank in some cases.
    The authentication information fields provide detailed information about this specific logon request.
    	- Transited services indicate which intermediate services have participated in this logon request.
    	- Package name indicates which sub-protocol was used among the NTLM protocols.
    	- Key length indicates the length of the generated session key. This will be 0 if no session key was requested.

Based on the included information, it appears that the event is a result of a failed network ("Logon Type: 3") logon or password change ("Caller Process Name: C:\Windows\System32\lsass.exe") for a user account that does not exist ("Sub Status: 0xc0000064").

I don't understand why the server appears to be failing to logon to itself, though. There are no relevant cached credentials on the affected servers.

This event is slightly different to all of the others that I've come across but, based on those, I've ruled-out the following common causes:

1. "Date and time sync between domain controllers". There is only one domain controller in each affected environment.
2. "Outdated or incorrect SQL credentials". SQL is not installed on each affected server.
3. "Expired security certificates in IIS". IIS contains no expired security certificates on each affected server.
4. Each computer objects' "objectSid" and "SID" properties are unique.

In one of the affected environments, there is a Windows Server 2012 R2 Essentials domain controller server and a Windows Server 2012 R2 Standard terminal / remote desktop server. Interestingly, the RDS server is logging the following events which are almost the exact opposite but do not appear to be logged at the same time as each other:

An account failed to log on.

    Subject:
	Security ID:		NULL SID
	Account Name:		-
	Account Domain:		-
	Logon ID:		0x0

    Logon Type:			3

    Account For Which Logon Failed:
	Security ID:		NULL SID
	Account Name:		SERVERNAME
	Account Domain:		DOMAINNAME

    Failure Information:
	Failure Reason:		Unknown user name or bad password.
	Status:			0xC000006D
	Sub Status:		0xC0000064

    Process Information:
	Caller Process ID:	0x0
	Caller Process Name:	-

    Network Information:
	Workstation Name:	SERVERNAME
	Source Network Address:	IPv6ADDRESS
	Source Port:		25228

    Detailed Authentication Information:
	Logon Process:		NtLmSsp 
	Authentication Package:	NTLM
	Transited Services:	-
	Package Name (NTLM only):	-
	Key Length:		0

    This event is generated when a logon request fails. It is generated on the computer where access was attempted.

    The Subject fields indicate the account on the local system which requested the logon. This is most commonly a service such as the Server service, or a local process such as Winlogon.exe or Services.exe.

    The Logon Type field indicates the kind of logon that was requested. The most common types are 2 (interactive) and 3 (network).

    The Process Information fields indicate which account and process on the system requested the logon.

    The Network Information fields indicate where a remote logon request originated. Workstation name is not always available and may be left blank in some cases.

    The authentication information fields provide detailed information about this specific logon request.
	- Transited services indicate which intermediate services have participated in this logon request.
	- Package name indicates which sub-protocol was used among the NTLM protocols.
	- Key length indicates the length of the generated session key. This will be 0 if no session key was requested.