Unexpected exception while `Site.validatePassword`

I have the following autocreated class: and I need to write a test class for that.

global class AutocreatedConfigSelfReg1587088360367 implements Auth.ConfigurableSelfRegHandler {

    private final Long CURRENT_TIME = Datetime.now().getTime();
    private final String[] UPPERCASE_CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.split('');
    private final String[] LOWERCASE_CHARS = 'abcdefghijklmnopqrstuvwxyz'.split('');
    private final String[] NUMBER_CHARS = '1234567890'.split('');
    private final String[] SPECIAL_CHARS = '!#$%-_=+<>'.split('');

    // This method is called once after verification (if any was configured)
    // This method should create a user and insert it
    // Password can be null
    // Return null or throw an exception to fail creation
    global Id createUser(Id accountId, Id profileId, Map<SObjectField, String> registrationAttributes, String password) {
        User u = new User();
        u.ProfileId = profileId;
        for (SObjectField field : registrationAttributes.keySet()) {
            String value = registrationAttributes.get(field);
            u.put(field, value);
        }

        u = handleUnsetRequiredFields(u);
        generateContact(u, accountId);
        if (String.isBlank(password)) {
            password = generateRandomPassword();
            System.debug('Generated random password: ' + password);
        }
        Site.validatePassword(u, password, password);
        if (u.contactId == null) {
            return Site.createExternalUser(u, accountId, password);
        }
        u.languagelocalekey = UserInfo.getLocale();
        u.localesidkey = UserInfo.getLocale();
        u.emailEncodingKey = 'UTF-8';
        u.timeZoneSidKey = UserInfo.getTimezone().getID();
        insert u;
        System.setPassword(u.Id, password);
        return u.id;
    }
}

The test code I’m writing:

@isTest static void testCreatetUser(){
    AutocreatedConfigSelfReg1587088360367 handler = new AutocreatedConfigSelfReg1587088360367();

    Account account = TestDataFactory.createAccount('HH_Account');
    String EXTERNAL_USER_PROFILE = 'Apply App';
    Profile profile = [SELECT Id, Name FROM Profile WHERE Name=:EXTERNAL_USER_PROFILE];
    String password = '';
    Map<SObjectField, String> registrationAttributes = new Map<SObjectField, String>();

    handler.createUser(account.Id, profile.Id, registrationAttributes, password);
    //TODO assertions
}

However, I’m getting an unknown exception while executing the line:

Site.validatePassword(u, password, password);

The exception is:

System.UnexpectedException: Salesforce System Error: 1762244533-9839 (-183289280) (-183289280)

Any direction on how to solve this would be much appreciated.