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Edit your site's configuration.php file. The $live_site should be empty. I believe it contains your old site's URL. If that's the case, empty it and save the file:

public $live_site = '';

Remember that you can see and change the Live Site URL in the Site Setup page when restoring a backup.

No worries! Everyone misses that setting. It's a confusing setting, after all.

The $live_site setting in Joomla! is there for a tiny fraction of misconfigured web servers which do not report the correct hostname. I say "misconfigured" because it's always due to a misconfiguration: the host is using a non-transparent proxy or SSL terminator which does not forward the HTTP Host header due to wrong configuration.

Unfortunately, the naming of this setting misleads people reading the configuration.php file into thinking that they must fill it in. This causes all sorts of problems, especially because it's a URL, not a domain name — even though it's there to work around an issue with the domain name being wrong on the stark minority of misconfigured servers. When it's set you may end up with infinite redirections between HTTP and HTTPS, broken CSS because the $live_site is a plain HTTP URL and you're accessing the site over HTTPS, etc.

Since most sites have this option set to an empty string and the impact of setting it to anything is not well understood among Joomla! users people are likely to blame literally everything else before someone tells them to take a look at $live_site. Now you know :)