I have CentOS release 6.3 (Final) and Centova Cast v3.0.4

I am running out of space in /usr and trying to move /usr/local/centovacast/var/vhosts to /home by:

1) Stopping Centova Cast:

# service centovacast stop
Stopping Centova Cast: cc-ftpd cc-web cc-appserver cc-control cc-imaged

2) Moving vhosts and creating symlink:

# mv /usr/local/centovacast/var/vhosts /home/
# ln -s /home/vhosts /usr/local/centovacast/var/vhosts

3) Starting Centova Cast:

# service centovacast start
Starting Centova Cast: cc-control setuid: Resource temporarily unavailable
Failed to drop privs- failed

What am I doing wrong?

I tried to remove the symlink and move vhosts back, but the result is the same, so I would keep it in new place.

Most likely you've still got some streams running the old/buggy ices that didn't reap its child processes correctly.  As long as any instances of the old version are running, they'll slowly eat up process slots until the kernel effectively says "no more for you" -- aka "Resource temporarily unavailable".

Make sure you're running the latest version of Centova Cast (update if needed) and then restart your ices-based streams (or at least their autoDJs) to get them running the latest, fixed version of ices.