This configuration check will report any invalid XML. The most common cause of this is international/special characters inside of your configuration in places where they are not supported. You must remove these characters from your configuration before proceeding with the upgrade, or else you will have problems, as your config.xml is not well-formed. Once you have upgraded to 2.0 you can put the characters back in descriptions, as they are properly supported in the new format. Config check output: Please fix the errors found above. It may help to view a config.xml backup file to see where the characters are exactly. Before proceeding with the upgrade, you should look over the upgrade guide on the doc wiki, which can be found here: https://doc.pfsense.org/index.php/Upgrade_Guide.