There are a wide variety of potential reasons for this. I can't comment to ask for more info, so I'll do the best I can here :)
One thing to do is to look for any messages that mention "afs" in the kernel log. Either look in /var/log/kern.log, /var/log/syslog, /var/log/messages, or the output of the
dmesg command. AFS may be complaining that it cannot contact certain servers, which could explain why something is hanging.
Another way to look into this further is to get a stack trace of all processes on the system. You can do this by running the following as root:
# echo t > /proc/sysrq-trigger
After running that, there should be a ton of information logged to the the kernel log (either /var/log/kern.log or others, as mentioned above). You'll need to provide that information to an OpenAFS developer so they can interpret it and say what's going on. You could send that information to email@example.com, or firstname.lastname@example.org. Be sure to include the version of OpenAFS you are running, as well as what platform (version of Ubuntu, or Linux kernel version). You should also provide the output of
cmdebug <hostname> if you can, where is the machine that is hanging.