Social II 2.0 includes system.css which doesn't exist in J4

templates/ja_social_ii/tpls/blocks/head.php

<?php // SYSTEM CSS $this->addStyleSheet(JURI::base(true) . '/templates/system/css/system.css'); ?>

You're right, Joomla 4 removed that file, I've shared with our team to update template.

Thanks for your report!