Campaign sign up form pushing not pushing data to Zoho CRM (Your Push to CRM has been stopped. Associated custom field has been deleted.)

Campaign sign up form pushing not pushing data to Zoho CRM (Your Push to CRM has been stopped. Associated custom field has been deleted.)

We are using the free plan for Zoho CRM and Campaigns. I currently have two Syncs using the sign up form. One is for members to send us their details which syncs with the Zoho CRM 'Contacts' module. This is working fine.

We have a second form which is for prospective members which I want to push to the CRM Leads module. This previously worked okay but now it's not working and I'm getting the error 'Your Push to CRM has been stopped. Associated custom field has been deleted.' 

So my questions are:

1. Is it possible under the free plan to have two forms that sync to the contacts and leads module.
2, If we can, what am I doing wrong :-)