Submit form: "An error occurred during the submit process. Cannot process content of type text/html".

I have created a PDF form, and submitting the data as html. The submission works but Acrobat / Reader would display an error "An error occurred during the submit process. Cannot process content of type text/html". The API is a cloud version and we cannot change the header type. I would like to add a simple popup/alert that tells the user ALL IS WELL. I don't want the user to be redirected. Please advise how can I add a popup?

Message was edited by: RICHARD POTGIETER