How can I fix the issue 'There was an error previewing the signature', shown at the lower right corner when trying to preview the signature?

This is usually caused when you copy and paste a 'Conditional IF', to fix it click on the HTML Source button and search for the '#{if...' piece of code and make sure there's nothing between 'if' and 'key', in the following example we show the correct statement for if with the work phone: #{if workPhone} ....