I'm getting a Forbidden error during iCloud sync

This usually means iCloud CalDAV has refused the request. Start with a few account checks:

• Make sure you're using an app-specific password rather than your regular Apple ID password.
• Confirm that your device's date and time are set correctly.
• Remove the iCloud account from Settings > Sync, then add it back with a freshly generated app-specific password.

If the error appears only with one specific list, create a new list, move the tasks into it, delete the original, then sync again.