I had an issue where emails were no longer being imported into WHMCS.

Child process of cagefs_virtual_address_pipe transport returned 255 (could mean shell command ended by signal 127 (Unknown signal 127)) from command: /bin/cagefs_enter

Turned out when WHMCS upgraded to version 7 some of the permissions were reset on the cron files.

Changing the /cron/cron.php file chmod to 755 got things working properly again.