Popup not showing pics

Sometimes when I click on a pic I get no pic on the popup, it shows only a loading gif.
Console error:
VM2126:2 Uncaught SyntaxError: Unexpected token N in JSON at position 1
at JSON.parse (<anonymous>)
at Function.n.parseJSON (jquery.js?ver=1.12.4:4)
at Function.a.parseJSON (jquery-migrate.min.js?ver=1.4.1:2)
at Object.success (iscwp-public.js?ver=1.4.8:103)
at i (jquery.js?ver=1.12.4:2)
at Object.fireWith [as resolveWith] (jquery.js?ver=1.12.4:2)
at y (jquery.js?ver=1.12.4:4)
at XMLHttpRequest.c (jquery.js?ver=1.12.4:4)