I was recently hired to solve a strange issue with one of my clients WordPress websites. On their site, they use a plugin called WP All Import to manage their property listings that come from an external CSV file. Everything seemed to be working properly until we wanted to automate the import process using a cron job. Every time the job ran on the server, WordPress would spit out an error:
invalid secret string
It turns out that they had installed a plugin called WP-Cron Control to manage cron in their dashboard. After disabling this plugin everything magically started working again. Long story short, make sure you properly configure all your plugins.