Source: Koen Roos blog

When you get a "Insufficient permissions" message when you assign a case, check that your current user''s permission roles has Read permissions on Queue.

When you get a message about user permissions, you can find out what permissions are needed for the action you want to do by doing the steps described on http://support.microsoft.com/kb/953962

Koen Roos, one of our experts, regularly blogs at Koen Roos blog.