Are you missing an underscore in the ENUM in your Create Variable action for $StringVariable?
Well, this is exactly how I have been using StringToEnum a couple of time already. Without any issue like you describe. Workaround is to place an errorhandler on the java-call and make the errorhandler do it the old-fashioned way: if $YourString = ‘Enkelvoudig’ then return enum_JaarrekeningType.Enkelvoudig, if… etc
Also try it with any other enumeration, even just for practice. See if it will work then.