Some useful links for group policy client side debugging

Group policy debugging:

How to enable it: http://blogs.technet.com/b/mempson/archive/2010/01/10/userenvlog-for-windows-vista-2008-win7.aspx

Enabling it for older OS's: http://social.technet.microsoft.com/wiki/contents/articles/4506.group-policy-debug-log-settings.aspx

How to understand the log files: http://blogs.msdn.com/b/richpec/archive/2009/07/20/userenv-debugging-line-by-line.aspx

Repairing local security policy: https://support.microsoft.com/en-us/kb/278316