The switch to Mojolicious with WeBWorK 2.18 broke the existing shibboleth module. It wasn't fixed until 2.19 (https://github.com/openwebwork/webwork2/pull/2443).
There are at least a couple of institutions using this, which requires setting up apache as a reverse proxy (which is covered in the 2.19 installation instructions).
The Shibboleth documentation on the wiki is very out of date, but hopefully this is enough to get you up and running. Please post again if you get stuck.