The CAS authentication module in WeBWorK hasn't been well maintained since none of the current developers have access to a CAS server to test against. There may be limited support available for issues with CAS right now.
There may have been a reason why CAS authentication required session management via key, but it's possible that that is no longer the case. There have been a lot of improvements to session management via cookie in the last few versions of WeBWorK, so I recommend trying it with cookies to see if it works.
In WW 2.19 the authentication method for the admin course is set separately from all other courses to allow higher security. If you copied your authen_CAS.conf file from your 2.15 installation you will need to add the following to that file:
$authen{admin_module} = [
'WeBWorK::Authen::CAS'
];
You should also check if there are any other configuration settings in authen_CAS.conf.dist that are missing from your authen_CAS.conf file.