'Non-human user detected' message when submitting a form

This is the honeypot being triggered which is an anti-spam measure built into Advanced Forms. It works by adding an invisible input that only bots will fill out, which then stop the submission. 

If this field is visible on front end form, there might be some CSS rules that are conflicting with Advanced Forms causing the input field to be visible.

The field has autcomplete="off" so it shouldn't be subject to autofilled values but if the field isn't visible and the message is still appearing, it may be something ignoring the autocomplete attribute and filling in a value anyway. If this is the case and the offending code/application can't be avoided or identified, deactivating the honeypot might be the best option.