On 10.11 Beta (15A226f) - in the Console I see quite a bit of these messages when Calendar is open - also I sometimes can't create new Calendar entries - tells me 'server is busy'. Not sure if other people have seen similar issues.
kernel[0]: Sandbox: mdworker(483) deny(1) file-read-data /Users/myuser/Library/Calendars/CBC104AF-F501-4AA5-8F21-30A1F7CF4566.exchange/Info.plist
CalendarAgent[297]: [com.apple.calendar.store.log.persistent.operation] [CalOperationQueue (x-coredata://68582EB3-B6CA-4851-9FA9-3EA17EC66A28/ExchangePrincipal/p21): Executing <CalExchangeGetAttachmentOperation: 0x7ffe638928c0> caused an exception [<SOAPFault 0x7ffe5e09b5d0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key Code..]
CalendarAgent[297]: [com.apple.calendar.store.log.persistent.operation] [CalOperationQueue (x-coredata://68582EB3-B6CA-4851-9FA9-3EA17EC66A28/ExchangePrincipal/p21): <CalExchangeGetAttachmentOperation: 0x7ffe66aab330> failed with error Error Domain=SOAPWebServicesErrorDomain Code=-997 "(null)".]
sandboxd[136]: ([484]) mdworker(484) deny file-read-data /Users/myuser/Library/Calendars/CBC104AF-F501-4AA5-8F21-30A1F7CF4566.exchange/Info.plist (import fstype:hfs fsflag:480D000 flags:240000005E diag:0 isXCode:0 uti:com.apple.ical.ics plugin:/Library/Spotlight/iCal.mdimporter - find suspect file using: sudo mdutil -t 37217335)
Can't recall having seen these issues with 10.10.x.
