It sounds like it is a permissions issue for the Calendar part.
I would re-create the BESAdmin account:
besadmin - Creating the BESAdmin Account
BESAdmin Account with Exchange Permissions - BESAdmin.ca
Dont forget the Send As:
Send As Permission - BESAdmin.ca
And then if you dont mine uninstall and re-install with the newly created service account. Or you can do a swich service account.
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