This does not seem to be a WordPress multisite issue.
This looks like a compatibility issue with the “wp-defender” plugin that you are using. It is trying to autoload a file from Site Reviews by its namespace instead of by its file path.
For example, the namespace of the file in the error is:
GeminiLabs\SiteReviews\Handlers
And the correct path to the Site Reviews file that is trying to be loaded in the error is:
WP_CONTENT_DIR/plugins/site-reviews/plugin/Handlers/RegisterTinymcePopups.php
But the “wp-defender” plugin is trying to load the file as:
GeminiLabs/SiteReviews/Handlers/RegisterTinymcePopups.php