Thanks for the advice. I checked the Chrome debugger (Ctrl+Shift+J on Windows in Chrome) on the “refused to connect” page, and it showed this:
Refused to display ‘https://[journal URL]/plugins/generic/pdfJsViewer/pdf.js/web/viewer.html?file=https%3A%2F%2F[journal URL]%2Findex.php%2FRICTD%2Farticle%2Fdownload%2F514%2F508%2F’ in a frame because it set ‘X-Frame-Options’ to ‘deny’.
Right now we are trying the config options outlined here to change the X-Frame-Options header. I’ll edit this comment with an update once we give it a try.
EDIT: We tried adding the X-Frame-Options header to the Apache config, and we still are getting the same error. Any ideas on what else could be causing this ‘X-Frame-Options’ to ‘deny’ error?