A form has been submitted. One of the fields, such as a text input field, contains forbidden markup, including < or > characters.
The client attempted to update a read-only field such as the process start time.
When a form is submitted, it is submitted as JSON. The attribute names area the field names; the attribute values are the field values.
Examine the form definition and make sure that your request does not attempt to submit markup within a plain-text field.