[Evolution] Constant "Error while Unknown background operation" errors?

[Paul Smith <paul mad-scientist net>]

I'm using Evolution 3.12.4 on Ubuntu GNOME 14.04.

I often see errors like this show up in Evolution (they appear in a
little space above the subject/preview windows):

Execution of filter 'Junk check' failed: Empty cache
file: /home/psmith/.cache/evolution/mail/1398703990 5795 13 myhost/folders/INBOX/cur/29/450023

FYI, the accounts use IMAP+, and I see it connecting both to my ISP's
IMAP server and also to gmail via IMAP.

They don't seem to hurt anything, except sometimes the preview pane for
the message is completely empty and I have to move away and move back to
see the contents.

Could this be related to the known mess around Evolution and Bogofilter
in Ubuntu?
https://bugs.launchpad.net/ubuntu/+source/evolution/+bug/1247366