With Checkmk 2.2, announced by Werk #14977, the usage of mod_auth_mellon was deprecated. This release now removes mod_auth_mellon.
If you still have mod_auth_mellon in your apache config, the apache service will not be able to start.
Errors are logged to var/log/apache2/error_log.
If you want to continue to use SAML you can do it in the Enterprise Edition via Setup -> Users -> SAML connections.
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