SQL Server returns error "Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON'." in Windows application

If your issue is with linked servers, you need to look at a few things.

First, your users need to have delegation enabled and if the only thing that's changed, it'l likely they do. Otherwise you can uncheck the "Account is sensitive and cannot be delegated" checkbox is the user properties in AD.

Second, your service account(s) must be trusted for delegation. Since you recently changed your service account I suspect this is the culprit. (http://technet.microsoft.com/en-us/library/cc739474(v=ws.10).aspx)

You mentioned that you might have some SPN issues, so be sure to set the SPN for both endpoints, otherwise you will not be able to see the delegation tab in AD. Also make sure you're in advanced view in "Active Directory Users and Computers."

If you still do not see the delegation tab, even after correcting your SPN, make sure your domain not in 2000 mode. If it is, you can "raise domain function level."

At this point, you can now mark the account as trusted for delegation:

In the details pane, right-click the user you want to be trusted for delegation, and click Properties.

Click the Delegation tab, select the Account is trusted for delegation check box, and then click OK.

Finally you will also need to set all the machines as trusted for delegation.

Once you've done this, reconnect to your sql server and test your liked servers. They should work.

First off: My problem isn't the exact same as yours, but this post is the first thing that comes up in google for the Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON' error at the time I wrote this. The solution may be useful to people searching for this error as I did not find this specific solution anywhere online.

In my case, I used Xampp/Apache and PHP sqlsrv to try to connect to an MSSQL database using Windows Authentication and received the Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON' error you described. I finally found the problem to be the Apache service itself running under the user "LOCAL SERVICE" instead of the user account I was logged in as. In other words, it literally was using an anonymous account. The solution was to go into services.msc, right click the Apache service, go to Properties, go to the Log On tab, and enter the credentials for the user. This falls in line with your problem related to SPN's as your SPN's are set up to run from a specific user on the domain. So if the correct SPN is not running, windows authentication will default to the wrong user (likely the "LOCAL SERVICE" user) and give you the Anonymous error.

Here's where it's different from your problem. None of the computers on the local network are on a Domain, they are only on a Workgroup. To use Windows Authentication with a Workgroup, both the computer with the server (in my case MSSQL Server) and the computer with the service requesting data (in my case Apache) needed to have a user with an identical name and identical password.

To summarize, The Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON' error in both our cases seems to be caused by a service not running and/or not on the right user. Ensuring the right SPN or other Service is running and under the correct user should solve the anonymous part of the problem.

I think there must have been some change in AD group used to authenticate against the database. Add the web server name, in the format domain\webservername$, to the AD group that had access to the database. In addition, also try to set the web.config attribute to "false". Hope it helps.

EDIT: Going by what you have edited.. it most probably indicate that the authentication protocol of your SQL Server has fallen back from Kerberos(Default, if you were using Windows integrated authentication) to NTLM. For using Kerberos service principal name (SPN) must be registered in the Active Directory directory service. Service Principal Name(SPNs) are unique identifiers for services running on servers. Each service that will use Kerberos authentication needs to have an SPN set for it so that clients can identify the service on the network. It is registered in Active Directory under either a computer account or a user account. Although the Kerberos protocol is the default, if the default fails, authentication process will be tried using NTLM.

In your scenario, client must be making tcp connection, and it is most likely running under LocalSystem account, and there is no SPN registered for SQL instance, hence, NTLM is used, however, LocalSystem account inherits from System Context instead of a true user-based context, thus, failed as 'ANONYMOUS LOGON'.

To resolve this ask your domain administrator to manually register SPN if your SQL Server running under a domain user account. Following links might help you more:
http://blogs.msdn.com/b/sql_protocols/archive/2005/10/12/479871.aspx
http://support.microsoft.com/kb/909801