“Please try after some time.” Error

I only want to capture email addresses and not send out any emails to anybody, not even admin. Also I get the read “Please try after some time.” notification when filling out the form with my email address and hitting SUBMIT.

I have single opt-in turned on and the email address still submits to my database but the user gets the error message on submit on the form.

I could hack the code so that the error message instead says “Success! You have been added to our newsletter mailing list.” so that when they are added to the database, despite the fact that the logic fails it still displays a successful message.

But what is the best fix for this? I think it has something to do with single opt-in.