Fix my sign-up form script with MailChimp API calls

Hi, I have a landing page here: [login to view URL] It uses a script to call the MailChimp API to do 3 things: 1) add the subscriber to the list 2) set the "form source" custom field to value "Landing Page" 3) add the subscriber to Interest group "10 Steps to Lucid Dreams" This was working fine for over 1 year, but in the last few weeks, it seems like the form has stopped working and now throws an error. Mailchimp Support have advised that a developer should fix the script. The scope of this project is to: 1) Troubleshoot the issue, working with me and MailChimp support as required 2) Fix the script so that above functionality 1-3 works perfectly again 3) Test the landing page and confirm sign ups are working again. Please note previous MailChimp API experience is mandatory. Note: access to production host will not be provided, so troubleshooting will be via myself. Thanks