Joomla Admin backend is broken

I tried to upgrade to Joomla 4. It failed. I restored from a backup on Siteground but it still doesn't work. I get this error message: "Error: Call to undefined method Joomla\CMS\Document\ErrorDocument::getWebAssetManager(): Call to undefined method Joomla\CMS\Document\HtmlDocument::getWebAssetManager()". I have disabled some plugins using PHPMyAdmin but still the back-end is not working. The front-end seems to work fine. www.cobo-arc.co.uk. Can anyone fix this quickly?