I was emerging a package and rebooted during it. After the reboot I tried to emerge the same package but it only halted saying that it was emerging the package. Using top I found a process "sandbox" that was running. I tried a sandbox --help but after showing the help it also halted. Looking inside the file /usr/lib/portage/sandbox I found a /tmp/sandboxpids.tmp beeing read or something. After deleting this file I was able to emerge. Perhaps something can be done to make this process easier. Thanks.
libsandbox.so has been moved to /lib instead of /usr/lib which should make this problem go away