And is this set somewhere in your web vhost configuration or only in the Nextcloud “SSO&SAML” Settings?
I use this CAS config:
CASVersion 2
CASLoginURL https://cas.domain.com/cas/login
CASValidateURL https://cas.domain.com/cas/serviceValidate
CASDebug On
<Location "/nextcloud/index.php/login">
AuthType CAS
AuthName "CAS"
Require valid-user
CASAuthNHeader HTTP_CAS_USER
</Location>
<Location "/nextcloud/index.php/apps/user_saml/saml/login">
AuthType CAS
AuthName "CAS"
Require valid-user
CASAuthNHeader HTTP_CAS_USER
</Location>