When I click a "Submit" button on the bottom of a completed form, I keep getting an error message next to an Adobe logo that says " At least one required field was empty. Please fill in the required fields (highlighted) before continuing."
There is no highlighting and no empty field. But there is plenty of frustration on my part.
One thing you can look for, before contacting the form maker, is whether there is a check box (like "accept terms & conditions", .that sort of thing). The form designer could have made a required check box, which could get this message, but with no highlighting.
Make sure the option to highlight the fields is enabled (via Edit - Preferences - Forms - Show border hover color for fields (yes, it's a very bad label). The required fields should then show up with a red border. If the field is hidden there's nothing you can do about it, though. I would then report this issue to the author of the form.