The error begins: "Fatal error: Uncaught Exception: Unable to open file /var/www/mediawiki-1.34.0/extensions/Favorites/extension.json ... and indeed the only .json files in the Favorites folder (from the current tar download) are composer.json, gitinfo.json, package-lock.json and package.json. Should one of those be symlinked as "extension.json"?
Note that was with the new way. The old spec of 'require_once "$IP/extensions/Favorites/Favorites.php";' doesn't error out on that nonexistent file, but instead, "
No such action
The action specified by the URL is invalid. You might have mistyped the URL, or followed an incorrect link. This might also indicate a bug in the software used by eiswiki.