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Since August 29th, many users have reported various login problems on Windows 11 and Windows Server 2025, and that includes both Windows 11 24H2 and the newer Windows 11 25H2.
According to Microsoft’s support page on the issue, the issue is caused by new security protections that are unfortunately causing Kerberos (the authentication protocol in Windows) and New Technology LAN Manager (NTLM) to fail for users with duplicate Security IDs (SIDs).
The issue can start occurring on relevant systems after installing either update KB5064081 (released on August 29th) or KB5065426 (released on September 9th). Symptoms include repeatedly being prompted for credentials, access requests fail even with valid credentials, shared network folders being inaccessible via IP address or hostname, failure to establish remote desktop connections, and errors in Event Viewer.
To permanently resolve this issue, affected devices with duplicate SIDs need to be rebuilt so that each SID is unique. Detailed information on how to do this is available on this Microsoft guide page.
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