I have enabled CAS authentication on my koha which is installed at library.domain.com The cas server is installed at cas.domain.com On the Login page, Koha generates the following link to login to the CAS server: https://cas.domain.com/cas/login?service=%2Fcgi-bin%2Fkoha%2Fopac-user.pl which is wrong, it should include its base URL in the service for the CAS server to determine the service and redirecting the user back, so the correct URL should be: https://cas.domain.com/cas/login?service=https%3a%2f%2flibrary.domain.com/cgi-bin/koha/opac-user.pl
FIX : Set OpacBaseURL : https://bywatersolutions.com/education/koha-url