Event ID - 5721

Event Id5721
SourceNetLogon
DescriptionThe session setup to the Windows 2000 Domain Controller name for the domain name failed because the Windows 2000 Domain Controller does not have an account for the computer computer name.
Event InformationExplanation:
This problem occurs if the computer name is the same as an existing user name. For example, if you upgrade a LAN Manager BDC to a Windows 2000 BDC with a unique computer name and then change the computer name to the replaced LAN Manager BDC, the above symptom occurs. When you create a Windows 2000 primary domain controller (PDC), the PORTUAS utility is run to convert the LAN Manager user account database (UAS) to a Windows 2000 security accounts manager (SAM) database. LAN Manager servers have a users group called SERVER, which contains the computer names of the servers as users. This group and its users were ported to the Windows 2000 PDC.

User Action:
To correct this problem: 1. Shutdown the new Windows 2000 BDC. 2. On the Windows 2000 PDC, remove the BDC computer name from the SERVER group in User Manager for Domains. 3. In Server Manager, remove the BDC computer name and synchronize with the domain. 4. After the synchronization is complete, add the BDC computer name and synchronize with the domain again. 5. Restart the Windows 2000 BDC. NOTE: If the domain becomes a Windows 2000-only domain, you may want to remove the SERVER group and the individual user names in the group. 
Fore more causes and resolution of this problem check the links to microsoft document given below. This problem also occurred if the computer name (netbios name ) configured in dns is not correct as the actual netbios name of the system.
Reference Links Event ID 5721 after Deleting Computer Account

Event ID 5721 When Upgrading LAN Manager BDC to Windows NT

BDCNetLogon Service Fails When Secure Channel Not Functioning

You May Be Unable to Establish a Trust Relationship Between Windows 2000 and Windows NT Domains

Catch threats immediately

We work side-by-side with you to rapidly detect cyberthreats
and thwart attacks before they cause damage.

See what we caught

Did this information help you to resolve the problem?

Yes: My problem was resolved.
No: The information was not helpful / Partially helpful.