SpamAssassin fails to start on CentOS 5.10

If you’re still running a CentOS 5.10 machine, you may notice that after recent updates SpamAssassin no longer works on your system. Trying to start it manually will get you a puzzling error:

Starting spamd: child process [12345] exited or timed out without signaling production of a PID file: exit 255 at /usr/bin/spamd line 2588.

Manually running sa-update also triggers an error (albeit more detailed)…