John,
The underlaying OS should have no effect on how cas runs.
Check your version of java against what is recommended for your version of cas (it may have changed with RHEL upgrade).
If banner is failing authn, it is not getting the attributes (in particular UDC_IDENTIFIER). Attribute retrieval happens during a callback (banner -> cas). (Unless you are using SAML2.)
The audit log should show what is happening / failing; otherwise, turn up logging in cas.
Upgrading major cas versions may involve changes to property names and service definitions (the class for the service). There will also be changes to key lengths for encryption / signing. Depending on your ticket store, this may require removing existing tickets
during the deployment (cas will no longer be able to read them).
If you can, set up a dev/test environment and run through the upgrade steps there.
Ray
P.S. With Modular we have this in our setenv.sh
JDK_JAVA_OPTIONS="$JDK_JAVA_OPTIONS \
--add-exports java.base/jdk.internal.ref=ALL-UNNAMED \
--add-opens java.base/java.lang=ALL-UNNAMED \
--add-opens java.base/java.nio=ALL-UNNAMED \
--add-opens java.management/sun.management=ALL-UNNAMED \
--add-opens jdk.management/com.sun.management.internal=ALL-UNNAMED"