I've seen this once a few weeks ago on one of our servers, although noticed it immediately as I was connected via KVM and so forced a reboot. Not seen it happen since so don't know what triggered it in particular at the time.
I reproduced this twice using the method below. The server hung on reboot twice, but not the third time. So this is not very consistent:
1) Set a user's shell to jailshell
2) Switch to the user and run a background process. For testing, I just had the user do a very large dd the first time, second time I ran a looping rsync. The user just needs to have a running process. The second time, I had this run as a cron job which obviously uses jailshell as well.
3) Issue a reboot from within the server. At init 6, it just stops - no errors or other output. I didn't get the same result as in the screenshot though. I had to issue a reset from the drac to get it to reboot.
Had the same problem. Server hangs on reboot. I had enabled jailshell once but had too many problems. Sites were randomly offline. So I had to disable it and it is disabled ever since. I don't know why I am having this problem although it is disabled.