When you are troubleshooting logon or Group Policy issues, you will very likely see event ID coming from userenv in your system log. What is userenv then?
Userenv.dll is a the Group Policy Engine that applies policies to your computer and your account. It calls different client-side extension to process different portion of your group policy. The most common CSE's include registry(administrative templates), security templates, folder redirection, software restriction, and so on.
Each CSE has a corresponding DLL to process the settings except registry CSE, which is process by userenv.dll itself.
If there is something wrong with your Group Policy application, possible causes include name solution not working, AD replication broken, FRS replication broken, and/or permission issue, you are very likely to see error reported from source "userenv" in Application Log.
To troubleshoot this kind of error, besides reviewing Event Viewer and collecting other basic information, most of the time you will need to enable User Environment debugging level to get more details.
Key: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon Value: UserEnvDebugLevel
Value Type: REG_DWORD
Value Data: 10002 (Hexadecimal)