This error can occur whenever there is content on your action (potentially including the letter content itself) which has unicode characters. Usually, the cause of this problem is copying/pasting data into the action, from some sort of outside text editor which supports unicode.
One handy online tool that could be used to troubleshoot this is:
In the top box, you can copy/paste the content that you suspect may have unicode within in. Then, click 'Encode'. The bottom box will then show you the same content, but it will add any special unicode characters (preceded by a "=" sign). You can copy/paste this content back into your action, then manually remove any of that "="-preceded coding, and replace it with text/formatting that you actually type directly into the action's setup. This should prevent any further unicode data from sneaking in.
In regards to preventing this from occurring in the first place, we suggest not copying/pasting content from text editors which may implement this type of special coding (hidden behind the scenes).