[Evolution] Bogofilter stopped working suddenly?

[Paul Smith <paul mad-scientist net>]

I'm using Evo 3.16.5 on Ubuntu GNOME 15.10.  I have 2 GMail accounts
and one IMAP (ISP) account configured.

Yesterday I noticed that I had a message in one of my GMail accounts
that wouldn't go away from Evo and if I tried to delete it Evo went
into some kind of constant download loop.  I decided to delete my cache
(rm -rf ~/.cache/evolution/mail/) and see if that helped.

It did help that problem but...

Now all of a sudden Bogofilter is no longer working.  Spam is not
filtered out of my inbox and I can see that the timestamp on my
bogofilter wordlist does not change when I fetch new email.  I can
still mark messages as junk in Evo and I can see that change the
timestamp on my wordlist.  So, it's just the ham/spam categorization
which is not working.

Just to be clear, I did check my IMAP account's settings and "check new
messages for Junk contents" is checked, and in the Junk tab of Mail
Preferences I have "Check incoming messages for junk" checked and the
Junk filtering software dropdown is set to Bogofilter.

I know, right, what could deleting my email cache have to do with
bogofilter?  I don't know, I'm just reporting what happened :).  Maybe
it's a coincidence, but I restart Evo fairly regularly, plus I have a
system at home with the same accounts configured and running the same
distro / version of Evo, and bogofilter still works there (so it
doesn't seem like a package update broke this).

Can someone suggest some debug/log settings I could enable to track
what's going on to see if there are errors etc. invoking bogofilter, th
at might help me see what's happening?