Calendaring Error “The operation failed. An object could not be found.

First thing I would try is to clean up the forms cache.
To do so, follow these steps:

1. On the Tools menu, click Options.
2. Click the Other tab, and then click Advanced Options.
3. Click Custom Forms.
4. Click Manage Forms.
5. Click Clear Cache.
6. Close all open windows, and then restart Outlook 2003, 2007, 2010 and O365.