I just ran into this on my server: graphics were loading fine until a couple of weeks ago, but this week we discovered they were being blocked.
The relevant lines in localOverrides.conf look like something that was leftover from an older version of WeBWorK (there's no way that it would have occurred to me to server images using a different port). Commenting them out has fixed the problem.
What I had:
$webworkDirs{htdocs_temp} = '/var/www/html/wwtmp';
#$webworkURLs{htdocs_temp} = '/wwtmp';
$webworkURLs{htdocs_temp} = 'https://webwork.uleth.ca:8443/wwtmp';
$webworkDirs{equationCache} = "$webworkDirs{htdocs_temp}/equations";
$webworkURLs{equationCache} = "$webworkURLs{htdocs_temp}/equations";
$courseDirs{html_temp} = "/var/www/html/wwtmp/$courseName";
#$courseURLs{html_temp} = "/wwtmp/$courseName";
$courseURLs{html_temp} = "https://webwork.uleth.ca:8443/wwtmp/$courseName";
# Location of MathJax script, used for the MathJax display mode.
#$webworkURLs{MathJax} = 'https://webwork.uleth.ca:8443/webwork2_files/mathjax/es5/tex-chtml.js';
But I tried using the defaults in localOverrides.conf.dist, and that doesn't work either.
Does anyone have any insight on the correct configuration when using SSL? (Maybe this would be solved by a wildcard certificate, but let's assume that's not an option.)