I suggest trying to run the cron command via a shell if you have shell access. If there are any errors, like missing Ioncube loaders, they should be displayed. If that's not an option, you can alternatively use GET or wget. See http://docs.blesta.com/display/user/Installing+Blesta step 4 Cron.
So, from this example, something like this may alternatively work for you..
GET -t 15m https://www.yourdomain.com/blesta/install/path/cron/?cron_key=CRONKEY