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BoxPlusSettings::setParameters(): Argument #1 ($params) must be of type JRegistry, Joomla\Registry\Registry given, called in /homepages/36/d457088995/htdocs/www/plugins/content/boxplus/boxplus.php on line 202

Third party issue. Content plugin by the name of boxplus. They are using a class name that was deprecated in Joomla 3.3.0 (early 2014) and removed in Joomla 5. They had 11 years to get the memo. What were they doing? Milking people for money without putting the darned work in?

I finally used Akeeba kickstart to restore a backup of my site. A new error appeared “Cannot instantiate abstract class Joomla\Module\Languages\Site\Helper\LanguagesHelper” and I no longer have access to the administration of my site.

You cannot restore a Joomla 4 site on top of a Joomla 5 site as per the documentation https://www.akeeba.com/documentation/akeeba-backup-joomla/restoring-backups.html#general-guidelines 

Do not try to overwrite one Joomla! version family with a different one. Overwriting a major version with another (e.g. restoring a backup taken on Joomla! 3.7 on top of a site running Joomla! 2.5 or vice versa) or between different minor versions (e.g. restoring a backup taken on Joomla! 3.7 on top of a site running Joomla! 3.6 or vice versa) will NOT work. Joomla! moves files around between minor and major versions. Since the backup does not delete files not present in the backup archive this will end up with Joomla! being "confused" and malfunctioning. In these cases you should delete the existing files and folders (except, perhaps, user generated content) before restoring the backup. You can safely restore a sub-minor (path-level) version on top of another. For example, you can safely restore a Joomla! 3.7.5 site on top of a Joomla! 3.7.3 site or vice versa.