ACF gives a syntaxError unexpected token

You’re missing quotes around the content of innerHTML.

If correctoutput is <div></div>, then you’re script looks like this:

 $('#correcto').innerHTML(<div></div>);

Which is invalid, because you’re missing quotes. innerHTML needs a string, which is in quotes in JavaScript. That first < is where your error is coming from. So you need to do this:

 $('#correcto').innerHTML('<?php the_field('correctoutput'); ?>');

Which would create:

 $('#correcto').innerHTML('<div></div>');

json_encode doesn’t cause the error because the result is a valid JavaScript type, which <div></div>, sans quotes, isn’t.