The specified object was not found in the store

Hi,

I am facing a weird problem since last 2 days. I have a mailbox, where I try to delete all mail/calendar items using EWS.
The cleanup code used to work fine until sometime back. But for the last two days, whenever I run this code, I am getting the following message.

The specified object was not found in the store.

The calendar item does exist in the mailbox (which I can verify by logging into Outlook and seeing it there)
This happens at random (most of the times i get the error, but sometimes it goes through)

I have been breaking my head to figure out what is going on... Do any one of you have any suggestions for me??
Is it something happening from Outlook side, where two servers are out of sync or something of that sort??

Please help.

M