This issue occurs because SLES 15 SP6 defaults to Java 21
As of now, SSPR (Latest version: 4.8) supports Java 11 only and does not yet support Java 19/21.
Self Service Password Reset System Requirements
Development is in the process of providing Java 19/21 support for SSPR in an upcoming releases.
Till then, please continue using Java 11 for running SSPR.