Error Approving Email - Token Key not found

Issue Description
In an attempt to approve an email, a 'Not Allowed' error message is being displayed with the following details:

{{lead.Email Address}} : Token key not found

Issue Resolution

1. Check that token syntax is correctly inserted with no missing left or right 'curly' brackets '{{' '}}'
2. Check that the field name of the token does not have any incorrect spelling (e.g. Email Address)
3. If the token syntax is correct and the field name of the token is spelled correctly, it is possible that the field name of the token contains a Unicode non-breaking space (e.g. between 'Email' and 'Address'). This would have been inserted if it was copied across from a platform that was encoded with Unicode (e.g. Microsoft Word) - To rectify this, delete the space in the token and re-insert it with a normal space using the spacebar on the keyboard. Once this is corrected, the email approval should work.