Unable to process your request. Please verify if the name and value is appropriate for the "email" parameter

Unable to process your request. Please verify if the name and value is appropriate for the "email" parameter

We have Mail Add On enabled.
We open the Emails tab in Zoho CRM, select an email , then select the CRM tab at the bottom right of the mail preview pane, then select the potentials tab and potentials come up that are connected to the email address of the account, however when we click on 'Associate' next one of the emails, we get a red warning that it was not possible to complete the action, with the message quoted in title: 
Unable to process your request. Please verify if the name and value is appropriate for the "email" parameter
See screenshot
Any idea what may be causing it?