Overview:
When creating an Event record in CRM for iPad, the following error appears instead of the Validation Rule error message: An error has occurred.
While the same works in CRM Online, the Validation Rule error messages display correctly on the Event Edit page.
Root Cause:
The Error Location field, on the Validation Rule, is set to Top of Page.
Solution:
Next to Error Location, enable the Field radio button in order to display Validation Rule messages on the iPad.
Related Documentation:
CRM Help Documentation: Offline Validation Rules