The site key check is against a hash of live_site. We don't actually see your domain. Just the hash. This hash needs to match or the check will fail. This means you need to consistently access your site the same way every time. I've checked your site keys and you have a site key for your site without www and with www. This means you're accessing it 2 different ways. So if you've the site key without www saved and you access your site with www it won't match. The same goes for http vs https. You can use htaccess mod_rewrite rules to ensure your site is consistently accessed.

Joomla Install from Web works fine, just not CB.

Joomlas doesn't need to confirm who you are so you can access paid products. They're just not the same thing in this situation.

The $live_site variable in configuration.php started causing issues with Joomla since 3.9.24. It literally made it impossible for people to login after that update. So it's set to an empty string now for all my sites. I think it has always been an empty string for this particular site.

live_site should typically always be empty. What I mean by the above is we ask Joomla for live_site and if that configuration is empty Joomla gives the current domain as live_site. The current domain is however you accessed the site. We check against a hash of that.

Please PM backend super user credentials for the site having issues and will take a look.

Ok, looks like something on your site is loading Guzzle 7 and Guzzle 7 removed ClientInterface::VERSION which is what we were using to determine what version of Guzzle was being used to allow backwards and forwards compatibility. Looks like they added a different constant to use. Have added a bug ticket to CB and will add a utility function for us to know what version of Guzzle is being used and will have to bug fix release 3-4 plugins to address this.

forge.joomlapolis.com/issues/8411