hi millardus
Its look like that user have corupted calendar entry
take a look at this KB
"CalICSSynchronizer::Synchronize - Synchronize failed: (0x80004005)" appears in the Windows Event Viewer
KB22637-"CalICSSynchronizer::Synchronize - Synchronize failed: (0x80004005)" appears in the Windows Event Viewer
This is deffinition of 8004010f error
MAPI_E_NOT_FOUND; 8004010f - 1. User accessing a resource that does not exist, 2. Attempts to resolve a recipient(s) that does not exist, 3. Improperly inputting a value of a field (property) on a Collaboration Data Objects (1.1, 1.2, 1.21) object where the field (property) does not exist.
follow kb and will fix ur problem