I'm getting a 500 error on Approve Post

ajax-error.js:35  SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
    at Function.parse [as parseJSON] (<anonymous>)
    at l (ajax-error.js:31:20)
    at p (ajax-error.js:95:21)
    at _ (rsvp.js:434:1)
    at y (rsvp.js:420:1)
    at m (rsvp.js:369:1)
    at invoke (backburner.js.js:282:1)
    at h.flush (backburner.js.js:197:1)
    at p.flush (backburner.js.js:358:1)
    at B._end (backburner.js.js:798:1)
    at B.end (backburner.js.js:589:1)
    at B._run (backburner.js.js:842:1)
    at B.run (backburner.js.js:624:1)
    at e.run (index.js:75:1)
    at t.error (ajax.js:148:7)
    at l (jquery.js:3223:31)
    at Object.fireWith [as rejectWith] (jquery.js:3353:7)
    at x (jquery.js:9629:14)
    at XMLHttpRequest.<anonymous> (jquery.js:9888:9)