(Solved) "proc_open(): fork failed - Cannot allocate memory"

Cause by:

Not enough swap space in the running server.

Solution: 

  1. Create new swap
    /bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=1024
    chmod 600 /var/swap.1
    /sbin/mkswap /var/swap.1
    /sbin/swapon /var/swap.1
  2. Run your command again.
  3. Remove swap
    /sbin/swapoff /var/swap.1
    rm /var/swap.1