The cron is called in oc-load only (if auto-cron enabled), or if you access to the file manually or by CLI. There isn't much mystery there. But it looks like the way that request is made (in background) doesn't fit your server well, so as Aficionado suggested. Disable auto-cron for a few days and check your logs. If the problem is still happening, then I don't have more ideas. If the problem have disappeared, then you should use your system's cron or an external one.