With the hypothesis stated in comment #7, I've restarted our MAAS server w/ PYTHONFAULTHANDLER=1 set so that, if it happens again, we can get a backtrace from regiond.
With the hypothesis stated in comment #7, I've restarted our MAAS server w/ PYTHONFAULTHAND LER=1 set so that, if it happens again, we can get a backtrace from regiond.