can not fork user shell: Resource temporarily unavailable

I am seeing this, too.

This does not occur with Ubuntu or Debian templates.

I checked file descriptor limits, open thread limits, etc. All seems well.

Anyone have any ideas?
 
We had the same Problem with centos6 based lxc containers

fixed it by changing the nproc to a higher limit than in /etc/security/limits.d/90-nproc.conf
standard is 1024 for non root users, which can caus trouble when using many nearly identical containers ;)