"Cannot fork" error messages

You must have set a limit for the maximum number of processes in /etc/security/limits.conf or perhaps some default is set in Ubuntu.

I had that set to 350 processes to prevent accidential fork-bombs. What drove me crazy - when my processes couldn't fork anymore, the number of processes I ran at the time was far from the limit.

Turns out the reason for that was, that not processes were counted, but kernel level threads.

ps -efL | grep ^$USER | wc -l 

indeed showed that I was close to the limit. The many tabs/windows I had open in chromium alone could amount to over 100 such threads - no wonder I hit that limit easily.

The problem ended up being this one:

https://bugs.launchpad.net/ubuntu/+source/php5/+bug/877894

fuser process started by cron job forking uncontrollably