One of our servers has started to have a problem with runit. Even after a
reboot we get this:

$ sv start ./service/unicorn/
timeout: down: ./service/unicorn/: 1s, normally up, want up

This has just started without (as far as we can tell) there being any
change to the server. I've even nuked the ./service/* directory so that it
will get rebuilt when the application is deployed (via capistrano - this is
a Rails app) but that does not seem to help.

The other 23 servers which are set up in the same way have no problem so I
am at a loss as to where to start looking.

Any idea of where I should look for clues?