This error is thrown in Mendix Studio Pro when you add a brace (which is the curly bracket: { ) into a text-field. Often because you want to show an example of a json-string to use with that rest-call. The text-field expects you to either 1) place a digit directly behind it, and a closing brace, and add a template variable or 2) add a second brace directly behind the first. This is the escape-sequence for the brace. Mind you, in case of a sample json-string, you probably have several opening braces that you need to escape.

ps. closing braces are handled correctly without the need of escaping them.