Usually, this message appears after the launched upgrade on the site with the non-working Cron Jobs on the server. But it needs to check the error log carefully for all possible reasons. And your current site has problems with DNS and isn't available for checking now.
UPDATE: one site of dear @Benny was upgraded fine, the only reason for this message was the stuck removal of the .bx_maintenance file in the root of UNA. It might happen if smth isn't good with the permissions of the server's user (not enough rights to delete/move of the script-generated file).