'Internal Server Error' error message when using the opt-in form in a Landing page.

Overview

You successfully created a Landing Page by adding a Form using either of the two available 'Confirmation' options: 'Redirect to another page' and 'Display confirmation message'. However, when you go to the Landing page, enter a valid(existing) email id and updating the information results in the error "Internal Server Error":

Information

This error gets displayed when the form is used to update existing clients.

1. The form in landing pages is to be used as an opt-in form for only new users and not to update existing contacts. 
2. For updating existing contacts, one needs to implement an Update Profile Enhancer.

Refer to this article for more information: Differences Between an Opt-in Form and an Update Profile Enhancer