On my Virtualmin console today, I see that the SpamAssassin filter is stopped. Not to mention, my inbox is full of spam. The problem seems to have started when I updated the SpamAssassin filter.
Clicking the start button results only in:
Failed to start service : The SpamAssassin filter server could not be
Trying to run
# /etc/init.d/spamassassin start
merely gives the cryptic message:
Starting spamd: child process  exited or timed out without
signaling production of a PID file: exit 255 at /usr/bin/spamd line
The solution, it turns out, is discovered by running spamassassin manually:
config: no rules were found! Do you need to run 'sa-update'?
Unfortunately, the bug reported here seems still not to be fixed as of August 2011. In particular, the error report from one of the internal scripts is not properly passed through to the user.
Fortunately, the solution is simple. As root:
and then you should be able to start SpamAssassin without error.