Using an Email DS Field for Reply to user

I love the CFF plugin and make good use of several of the add-ons to this plugin.
However, I have a problem with using the Email DS field as the email to which to send a response message.
The form is only intended for logged-in users of our website, so we already have their email address. When I add an Email DS field with the appropriate settings to get the email of the current user it populates correctly. So far so good.
The problem occurs when the user hits the Next button. An error message appears briefly (to brief for me to read it) and the form does not advance to the next page.
When the Next button is clicked a second time, it works and advances to the next page of the form. So it seems that the first Next page button click validates the value in the Email DS control.
My short-term fix is to put a message on the page that tells the user to press the button a second time but this is a bit messy. I feel there should be a way to avoid the error and go to the next page without the error message but I am not sure how to make this happen.
One other thing to add is that I have the Email DS set to read-only because I don’t want users to change their email address from the one they registered with us when creating their account.
It would be wonderful if you could tell me how to resolve this issue. Thank you!