Do you have the open files limit set sufficiently high?
ulimit -n
should show 16k or higher.
That shouldn't be the problem - nofile is set to 32k in the upstart script.
My setup:
Ubuntu 12.04 LTS (kernel 3.2.0) w/Python 2.7.3; latest Electrum server from git; leveldb 1.9.0; plyvel 0.8