Microsoft Outlook email - Dissapearing Calendaring Items

We have some users experiencing calendar items that randomly
disappear. I think it is related to the errors that we get in the Event log:

Calendaring agent failed with error code 0x8004010f while saving
appointment.

The data of the Event Log is, for example:
HrSaving
0008: 41 70 70 74 3a 3a 48 72 Appt::Hr
0010: 43 68 65 63 6b 50 61 74 CheckPat
0018: 74 65 72 6e 20 66 61 69 tern fai
0020: 6c 65 64 2e 20 4d 61 69 led. Mai
0028: 6c 62 6f 78 3a 47 4a 6f lbox:GJo
0030: 68 6e 73 74 6f 6e 40 6d hnston@m
0038: 65 72 69 64 69 75 6d 2e eridium.
0040: 63 6f 6d com

What can I do to track down the reason behind these random Calendar
items disappearing?

Platform: Exchange 2007 SP1 Update Rollup 2 on a two node CCR cluster
running Server 2003 SP1 Enterprise 64 bit