tcpdump: can't create rx ring on packet socket: Cannot allocate memory

tcpdump: can't create rx ring on packet socket: Cannot allocate memory

I've been using tcpdump without any issues before this error popped up. I
use the following two commands with variations as soon as my server is
attacked so the packets automatically get dumped if the packet rate is
high.
tcpdump -i eth0 -p -nn -s0 -c 2000 -w $dumpdir/dump.`date
+"%Y%m%d-%H%M%S"`.cap
tcpdump -nn -s0 -c 2000 -w $dumpdir/dump.`date +"%Y%m%d-%H%M%S"`.cap
The following error shows up in console as soon as I'm under attack:
tcpdump: can't create rx ring on packet socket: Cannot allocate memory
Using the command manually works so my guess is that something gets
overloaded in the event of an attack. Here is my RAM usage though:
root@x:~# free -m
total used free shared buffers cached
Mem: 2004 1255 749 0 1 29
-/+ buffers/cache: 1225 779
Swap: 2047 1095 952
And currently, it is much lower than it used to be when the command worked.