Salesforce For Outlook error 'This event can't sync because it was converted into a recurring series in Outlook'

Salesforce for Outlook (SFO) is a Salesforce application that syncs contacts, events, and tasks between Microsoft Outlook and Salesforce. This article explains a sync failure that occurs when an Outlook event type is changed after it has already been synced to Salesforce.

Issue

The Salesforce for Outlook system tray icon changes to a red exclamation mark indicating that the sync has failed. When you double-click the icon in the System Tray, you will see the error message:

'This event can't sync because it was converted into a recurring series in Outlook.'

Root Cause

This error occurs when a synced single event or meeting invite is converted to a recurring event, or a synced recurring event is converted back to a single event. Salesforce for Outlook does not support this type of conversion after sync has occurred.
When a single event is promoted to a series, the Sync.log file records:

[Event] An error occurred in converting OL record <name of the event> of type Event. Error: Sfdc.Synchronization.InvalidUpdateOperationException: Detected a previously-synced single event that was promoted to a series in outlook

When a series is demoted to a single event, the Sync.log file records:

[Event] An error occurred in converting OL record <name of the event> of type Event. Error: Sfdc.Synchronization.InvalidUpdateOperationException: Detected a previously-synced series that was demoted to a single event in outlook

Reproducible Scenarios

1. Create a single event or calendar invite in Outlook and sync it to Salesforce. After syncing, change the event pattern in Outlook to a recurring series. The sync will fail on the next attempt.
2. Receive a meeting invite in Outlook, accept the invitation, and sync the meeting to Salesforce. The organizer then changes the event into a recurring series and sends an update. Accept the update in Outlook. The sync will fail on the next attempt because the event type has changed.