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Event ID 1109: The user account is in a different forest than the computer account

Windows Event ID Lookup: What Windows event IDs mean: 4625, 4624, 4740, 6008, 41, 7045, 1102 and 3,000 more, with Windows' own message text, the fields and what to check.

Look up a Windows event ID from Event Viewer, a SIEM alert or a log export: what it means, the fields it records, and what to check. Many IDs are used by more than one source (event 41 is both a Kernel-Power crash and a boot error), so every definition is shown, the one people usually mean first. The message text is Windows' own, read from the event providers of Windows 10.0.26200.0, 2026-10-28; the notes are ours, based on Microsoft's security auditing reference.

Try: 4625, 4624, 4740, 6008, 41, 1074, 7045, 1102, 4104, 1149

The user account is in a different forest than the computer account.

Event 1109
GroupPolicy
System log
Warning
Message and fields (as Event Viewer shows them) 
The user account is in a different forest than the computer account. The processing of Group Policy from another forest is not allowed. Group Policy will be processed using Loopback Replace mode. The scope of the user policy settings will be determined by the location of the computer object in Active Directory. The settings will be acquired from the User Configuration of these policies.