Citrix XenApp 6/6.5
Windows Server 2008 R2

If users are getting an access denied even though they are a part of the Users group, this may be related to the MaxTokenSize key.  We've seen this issue with users who are members of many AD groups and it seems that the enumeration cannot process the correct group that gives them access to the application.

Modify the registry on the Citrix server to allow for this setting:

Entry: MaxTokenSize
Data type: REG_DWORD
Value: 65535 (Hex)

Reboot the server and the user should be able to login.