Check the cpanel error logs, if you get the following. Have a smile, because fix is simple.
Out of memory during “large” request for 4198400 bytes, total sbrk() is 36796416 bytes at cpanel.pl line 4686.
In WHM/Tweak Setting, find the option
‘The maximum memory a cPanel process can use before it is killed off (in megabytes). Values less than 128 megabytes can not be specified.’
Increase the value and check again if it help.
Posted by We3cares 