Solution above did not do the trick.

During parsing blockchain files the number of unclosed files increase.
Running the command below gives me a higher number during (re)building:

The code and (possible) solution is mentioned on:
http://stackoverflow.com/questions/16386678/too-many-open-files-python-multiprocess-tcp-server
Mentioned solution: "The problem solved. File descriptor assotiated with socket must be manually closed. os.close(fd)"

Hopefully you can reproduce this, or maybe it is only a problem in Ubuntu 12.04, 32-bit.