Possibly because of low memory.

You can check the cpanel logs for the errors from: 

# tail -f /usr/local/cpanel/logs/error_log

If it says that a process is terminated because of low memory.

You can change the memory limt value for cpanel from the WHM >> Tweak settings

“Max cPanel process memory” 

and set this to unlimited and save. Try re-signing and it should work.

If signing still fails, please open a support ticket with us, and we will be happy to check this for you.