We had the same problem and it turned out to be another plugin which was handling cron wrong. Basically, it was detecting if cron was running and exiting early, but unintentionally it was exiting the entire PHP process rather than just its own code.
If you can, SSH into the server and run~
grep -rnw '/path/to/wp-config/plugins/' -e 'die'
grep -rnw '/path/to/wp-config/plugins/' -e 'exit'
and try disabling any plugins that have files listed.
The solution might be to replace