The “iCal server responded with an error” message typically occurs when there is a problem syncing or updating calendar data with an iCal/CalDAV server, especially in macOS or iOS Calendar apps.
This can happen from corrupted events, recurring event issues, or server communication errors.
Common Causes and Fixes
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Corrupted or Conflicting Calendar Events:
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Sometimes recurring events or events associated with deleted accounts cause sync errors. For instance, a recurring event from a deleted Google account continuing to send requests triggers this error. Fix by logging into your calendar service web interface (Google Calendar, iCloud, etc.) and delete or edit problematic recurring events.
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400 Error from CalDAV Operation:
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This indicates a bad request due to calendar data issues. Clearing out problematic events often resolves this.
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Email Provider Changes:
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Some users experienced this when email providers like Verizon/AOL/Yahoo changed back-end email handling. Switching to “Basic” mail mode in webmail settings restored functionality.
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Sync Issues with iCloud or Third-Party Servers:
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Try removing and re-adding the calendar account to force a fresh sync.
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Steps to Fix
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Log into your calendar service (Google Calendar, iCloud, etc.) on the web and check for suspicious or old recurring events, especially from deleted accounts. Delete them if found.
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On your device, try removing then re-adding the calendar account causing issues.
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Clear cache or settings related to calendar apps, if possible.
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For mail providers (AOL/Verizon/Yahoo), switch to “Basic” mail mode on their webmail portal.
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Restart your device to clear any temporary sync errors.
This should resolve the “iCal server responded with an error” messages related to calendar sync and recurring event conflicts.
