?cmd=contents&module=calendar error 500

Giving permission to calendar folder by

set-MailboxFolderPermission user:\calendar -User manager

not works when the Manager tries to open user's calendar by url

https://server/owa/user@smtp.com/?cmd=contents&module=calendar

Manual from Exchange 2007 (either should work for Exchange 2010):

http://exchangeshare.wordpress.com/2009/06/15/how-to-open-shared-calendar-in-owa/