Steps to reproduce:

1. Precondition: Zlib support in PHP is not enabled
2. Navigate to Administration -> Authentication and configure SAML settings.
3. Logout and try to login using "Sign in with Single Sign-On (SAML)".

Result:
500 error.
From PHP log:

PHP Fatal error: Uncaught Error: Call to undefined function OneLogin\Saml2\gzdeflate() in <...>/vendor/php-saml/src/Saml2/AuthnRequest.php:188

To solve the problem in SUSE Linux, separate package php-zlib must be installed.
When installing PHP from source, it should be configured --with-zlib[=DIR].

The error will not appear if compression of SAML requests and responses (enabled by default) is disabled using the following settings in zabbix.conf.php:

$SSO['SETTINGS'] = [
    'compress' => [
        'requests' => false,
        'responses' => false
    ]
];