please check your server error log and see if there is any error.
Also as you said there is an error in your cron events overview, can you please check from which plugin and/or theme that error came from? Can you deactivate that then try?
If still not working please deactivate all plugins except this and change theme to unmodified default theme then try. If require you can activate cron plugin to manually run cron. If works then change your theme to your current theme and activate plugins one by one and try. If you found any conflict let me know.
Also go to Front End PM PRO > All Announcements and see “Email Pending” column to see if that showing pending emails.