The class is defined in the file $QA_BASE_DIR/qa-include/Q2A/Plugin/PluginManager.php. In your case that would be /storage/ssd3/173/19637173/public_html/qs/qa-include/Q2A/Plugin/PluginManager.php. The file is part of the Q2A base install. If it's missing from your installation you need to investigate why that is and/or restore it from the install archive. If the file is present but still cannot be found it could be a permissions issue or a problem with the PHP include_path. Your webserver error log should provide more information.