Hi,
I have the same issue, but not only in WHMCS. In fact, any of my cron jobs on any server or account is running.
cat /var/log/cron
the cron log is:
Jan 12 14:00:52 forseti crond[77]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 68% if used.)
Jan 12 14:00:52 forseti crond[77]: (CRON) INFO (running with inotify support)
Jan 12 14:01:01 forseti CROND[992]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:01:01 forseti CROND[993]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:02:01 forseti CROND[1091]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:02:01 forseti CROND[1092]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:03:01 forseti CROND[1431]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:03:01 forseti CROND[1430]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:04:02 forseti CROND[1540]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:04:02 forseti CROND[1541]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:05:01 forseti CROND[1640]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:05:01 forseti CROND[1639]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:06:01 forseti CROND[1751]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:06:01 forseti CROND[1750]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:07:01 forseti CROND[1857]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
Jan 12 14:07:01 forseti CROND[1856]: (CRON) CAN'T FORK (child_process): Resource temporarily unavailable
I research for several different causes for this output, most of the solutions is increasing the security limits or UserTasksMax but doesn't work for me.
Do you have configured or personalized some security limits?