This occurs because CloudFlare on your server is returning status code 520 for some Javascript files occasionally, in this case some loaded via the concatenator wiki_js.php, which breaks Javascript on the page. The behavior is inconsistent and you can successfully load the file if you manually open it. The tile images load successfully during the attempts where CloudFlare returns a normal status. You may want to review your CloudFlare logs when the files fail to load. Possibly you are rate limiting yourself too hard, or another issue. This does not appear to be a bug in VaultWiki, because the image behavior is correct when the server correctly returns the Javascript files that are requested.