Just to be sure, what version of ACF are you using. I’m assuming that it is ACF4 (the free version) because of the path you’ve given for the error. The file you’re referring to does not exist in ACF5.
The only way that you get an error on line 338 is calling the ACF function the_field(). 338 is the last line of that function where the value is echoed. For some reason when ACF is attempting to get the value of the field it is causing a WP Error. In order to track down what’s causing the error you need to start by looking that the line in your template that’s causing the error.
Take a look at the page source of a page where the error happens. Look at the HTML that’s being generated before the error. Track down where that code is being generated in your template file. The call to the_field() will be somewhere after that HTML is generated.
