There was a bug in the file Shibboleth.pm, which caused this error. See: https://github.com/openwebwork/webwork2/issues/1844
Alongside this bug, the documentation for using Shibboleth was incorrect in the first place. Please see the revised documentation that is now posted to the Wiki, which includes a more clean and streamlined regex entry.
I worked with my campuses senior security personnel to help debug this issue in our instance, and to propose a hotfix for WW2.17.
WW2.16 may still have this bug.
Known working fine in WW2.14.