Validation errors may appear as a result of hiding required fields using a client-side script.
Depending on the field type and the validation that is causing the issue, the specific field that is causing the problem may not be referenced explicitly.
A typical example of this issue is where an error message appears stating that an invalid datetime value has been entered, but the field is not highlighted.
One method of troubleshooting this issue is to search the Custom_Screens table for recent updates to fields on that screen. A field may have been added to a screen, or made required, causing the issue.
Once a list of fields is obtained, further checking can be carried out by checking the Field Security options in Administration | Customisation | <Entity> | Fields, or by checking for Create or Validate (server-side) scripts on the screen's fields.
Typically, when this error has been displayed, the screen has been hidden from the user interface, generally, through use of a client-side script. This causes the field not to be highlighted when the error is thrown.