Since my recent SL viewer update I've noticed my mirrors are inactive upon login. Thinking it was turned off by the update I went to look at preferences and found mirrors to be set on and the associated options to be correctly configured.
Setting mirrors off and saving, then setting the mirrors check bob on again (and saving) restored the mirrors.
However once again, after logging off, and back on (the next day) mirrors are once again inactive. And once again, mirrors are checked on in Preferences. Toggling the Mirror checkbox restores their operation until next time.