Did you receive these kinds of email, then you should check your disk space of your server especially the disk space usage of /tmp folder. it is not a root level compromise. Also It happens when you have Nginx Admin – cPanel nginx automated installer Plugin installed.
Sometimes servers can have a lot of tmp files and if tmp gets filled it stops a lot of things from working services. Disk space issues are quite common in cPanel accounts and servers. It can be mainly because of the website users eats up the entire disk or the log files fills up or the backup fills up the entire folders.
Attempts to create new directories or files whose file-names begin with numbers have failed. This is indicative of a root compromise of the server. The exact error encountered was: Failed to create directory /dev/null: No such file or directory
#- Delete some files from
tmpwatch -c 48 /tmp/nginx_client
or manually remove nginx temp file from /tmp/nginx_client and restart nginx server.
Check your /tmp folder disk space.
root@server1 [~]# df -h Filesystem Size Used Avail Use% Mounted on /dev/sda1 40G 18G 20G 48% / tmpfs 499M 0 499M 0% /dev/shm /usr/tmpDSK 1.5G 240M 1.2G 17% /tmp
check inodes on your partition.
root@server1 [~]# df -i Filesystem Inodes IUsed IFree IUse% Mounted on /dev/sda1 2621440 331517 2289923 13% / tmpfs 127549 1 127548 1% /dev/shm /usr/tmpDSK 99840 52645 47195 53% /tmp
You should check whether any partition have 100% disk space and clean tmp properly.