We have received reports of iCal sync responding with the following error message when trying to schedule or modify appointments in iCal after upgrading to Mac OS 10.8:
The server responded with 400 to operation CalDAVWriteEntityQueueableOperation.
To resolve the issue, click iCal (in upper left corner) > Preferences > Alerts. Select your zMailCloud account set a default alert to anything other than none (15 minutes before, for example) in the Events field. Also make sure "Use these default alerts on only this computer" is NOT checked. Please verify that the setting change took effect after setting Alerts by navigating back to iCal > Preferences > Alerts.