I’ve seen several features of cPanel appear to malfunction, and upon reviewing /usr/local/cpanel/logs/error_log, I’d see something similar to this:
Out of memory during request for 2180 bytes, total sbrk() is 130234368 bytes!
Common places this has been known to occur:
In Webmail (Horde and Roundcube) when opening large attachments
Using cPanel’s perl module installer
You could legitimately be out of RAM, but most likely the cause is cPanel’s internal memory limit. You can raise this in WHM > Tweak Settings:
“The maximum memory a cPanel process can use before it is killed off (in megabytes). Values less than 128 megabytes can not be specified. A value of “0″ will disable the memory limits.”
Or you can adjust the maxmem setting in /var/cpanel/cpanel.config.