Directly after startup, I received:
SQL> startup ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instance ERROR: ORA-28040: No matching authentication protocol
After some checking, I found and offending entry in $TNS_ADMIN/sqlnet-ora:
SQLNET.ALLOWED_LOGON_VERSION_SERVER=12A SQLNET.ALLOWED_LOGON_VERSION_CLIENT=12A
This may look ok, but it really needs to be
SQLNET.ALLOWED_LOGON_VERSION_SERVER=12a SQLNET.ALLOWED_LOGON_VERSION_CLIENT=12a