exchange 2003 issues -calendar entries deleting themselves

I am having an issue with exchange 2003 on sbs2003.  Users within a department share calendars and all have their exchange accounts set up on their smartphones.

The problem started about 2 weeks ago, and there were no major issues before that.  They are having an issue where calendar entries are deleting themselves.  Also, calendar entries made on their mobile devices are not showing up in outlook on their desktop.  All the users are have iPhone 4's.  There are about 10 users in the department who all share, and work out of each others calendars.  
All the affected users have error 3005 in the server log, with refrence to their user name.
The following are error messages in the event logs on the server related to the specific users having the problems, and also some general errors and warnings with exchange:

MSExchangeISMailbox started.  Consecutive ma-open calls are failing with error 3051.
Event 2000
User N/A

Server Activesynch
Event 3005
User:  department user with calendar event deletion issues
Unexpected exchange mailbox server error.  Status code 500.  Verify the exchange mailbox server is working correctly.

I have checked online for solutions to these errors, and by description of the issues and devices, and have not come up with a difinitive answer.  What is your assessment of the situation?
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AE_JBAuthor Commented:
The links help, but the solutions seem to involve making system wide changes which I would want to avoid if possible.  
I have re posted here:

to see if a localized solution can be found.
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Windows Server 2003

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