It's because PHP 8.1 is newer than the CMSB version you have and PHP updates are not fully backwards compatible so when you upgrade PHP it can start reporting errors.

Your options would be:

• temporarily downgrade to PHP 7 to make the backup
• use another tool such as your website's control panel or phpmyadmin to backup the database
• or just upgrade the CMS without making a database backup first

If it's possible or easy to make a backup, we do recommend that, but upgrades are generally pretty safe and I can't recall a recent issue where I needed to use a MySQL backup after an upgrade issue.  Additionally, your web host might have automatic backups that you can access if there was an issue.

Hope that helps, let me know any questions.

The errors are also appearing on the client website pages (but luckily are being hidden behind a header bar on all pages).

Do you have any ideas what the issue is and how to solve the login?

Thanks
Adrian