** Using WeBWorK **

It looks like the lighttpd server (which often uses port 81) is not working or serving images. Take a look at the part of the instructions configuring the lighttpd server and see if there are problems there.

The other possibility is that the graphics files (equations or .gif images) aren't being created, probably because the webserver doesn't have permission to write into some directory. In that case you should also see other error messages appearing in the log files.

Check first to see if the graphics files are where they are supposed to be, then check the lighttpd server -- you can try to access the graphics images directly using the URL you have listed above to check if the lighttpd server is working and configured properly.