The problem is because the besadmin service account cannot write into the calendar. The besadmin account uses either the default user or the anonymous user (or both) to write into a user's mailbox.
To see this in Outlook:
Right click on Calendar
Go to the Permissions TAB
The default permissions should be
- default on None
- Anonymous on None
The awkward thing is that a user can remove the anonymous account, but will never be able to put it back. Only an administrator can put it back (see the link). When this problem occurs, the user will very likely not have the anonymous account in his/her permissions list. A workaround (quick dirty, but not nice) is to add another account with enough permission to the list.
There are two ways to solve this
1) Download details: Microsoft Exchange Server Public Folder DAV-based Administration Tool
Download PFDAVAdmin and then run it (after running backups) and go to the User, Top of Information Store. Right-click Top of Information Store and choose Propagate Folder ACE's. Make sure to choose Add/Replace as the option. This will re-add Default and Anonymous to the Calendar and the other folders.
2) Backup the user's mailbox to a PST. Delete the mailbox en create a new one. It should have all the default mailbox permissions. Restore the mailbox using the PST.
Hope this helps.