Code:
family:~# vserver father start
vcontext: vc_ctx_create(): File exists 

An error occured while executing the vserver startup sequence; when
there are no other messages, it is very likely that the init-script
(/etc/init.d/rc 3) failed.

Common causes are:
* /etc/rc.d/rc on Fedora Core 1 and RH9 fails always; the 'apt-rpm' build
  method knows how to deal with this, but on existing installations,
  appending 'true' to this file will help.

Failed to start vserver 'father'
In this case, the typical error message is vcontext: vc_ctx_create(): File exists.

After a quick diag, I noticed that it's caused by duplicated context id, meanwhile this vserver is running:

Code:
family:/etc/vservers# grep 40004 */context
father/context:40004
yetanother/context:40004
It's really a stupid error

More detail is available here: https://support.mayfirst.org/ticket/1842