Solution: We can open your Magento database using a database management tool like PhpMyAdmin.
Search for the core_config_data table and change the value of the web/secure/base_url setting to use HTTP.
Clear the cache and the session records from the /var/ Magento folder and load the admin area.
Then you should be able to log in the admin area and adjust the SSL configuration.
Alternatively, contact your web hosting provider to reinstall the new SSL certificate. If it is issued for the same domain as the old one the online shop will continue working correctly. Otherwise, you should change the secure base URL in the database as explained above.
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