Thank you, but the issue was with the SEO & Index Tools add-on. I tried disabling all add-ons by setting $config['enableListeners'] = false; in config.php, and the site worked fine. Then, I reactivated the add-ons one by one until I found out that the problem was with the SEO & Index Tools add-on.
After upgrading to version 2.3, this is what happened on the website:
Some menus in the administration can be accessed, but 90% of the website is not functioning.