Outlook 2010 Calendar issue - can't delete items of shared calendar, but can move it of

Question

So I'm deploying Office 2010 to our group and our office assistant/receptionist helps manage several calendars and he was able to do so witout problem after the upgrade from Outlook 2003 to Outlook 2010. But the other day out of the blue he started to not
be able to add meetings to peoples calendar, but that now works in part to the /cleanfreebusy switch, but he is not able to delete meetings from other peoples calendar. He keeps getting the following error mesg: "Could not complete the deletion.
The items may have been already deleted or moved. "
He does have full editor access for the calendars. The other users are still using Office 2003, except for two (who are using 2010), but he has the same issue with their calendar also. Now here is the really strange thing, he is able to drag and move (holding
the shift key down) the appointment from the users calendar to his and then it's gone from the other persons calendar, then he is able to delete it from his calendar without any issues.
I have removed and recreated the Outlook exchange mail profile. The meetings he puts on peoples calendar does show on those peoples calendar. I've run every Outlook command
line switch I could find.
Help!

Answer 1

Dear David,
Based on my research, this issue might be related to many factors, such as some third-party add-ins’ conflicts. At this time, I suggest checking the issue by
starting Outlook in safe mode.
Start the Outlook program in safe mode
==============
1.      
Click Start, point to All Programs, and then point to
Microsoft Office.
2.       
Press and hold the CTRL key, and then click
Microsoft Outlook.
If the problem does not occur in the safe mode, this issue might be related to some third-party add-ins in the Office program, we can try to disable them. Normally,
you could do the following to disable the conflict add-ins in your Outlook program:
Disable add-ins
a.    
Click
File menu à click Options à
Add-in à click Go button in the Manage:
Com-in Add.
b.   
Check if there are any add-ins,
clear the checkbox to disable them.
c.    
Close the Office program and
restart it.
d.   
Add one check back each time to the list of Add-In,
restart the Office program, and repeat the above procedure. Once the issue reappears again, we can determine which add-in causes this problem and then disable it.
Best Regards,
Sally Tang