CQ logs claim to be scheduling 8 builders through buildbucket every ~1 minute, followed by fetching exactly 0 builds back. Gerrit UI also shows 0 builds for a CL, so I'd guess something goes wrong with scheduling.

You can also see what's going on https://chromium-cq-status.appspot.com/recent#issue=585413,patchset=2,codereview_hostname=chromium-review.googlesource.com

buildbucket logs claim that all of these builders aren't defined in their respective buckets.

Builder u'crashpad_try_win_x86_rel' is not defined in bucket u'master.client.crashpad'

Dirk, I think you broke it :)
https://chromium.googlesource.com/crashpad/crashpad/+/20b6f795897737dd0eb63378881585125d966261

Meanwhile, CQ should have detected and propagated errors from buildbucket and aborted attempt. For this, issue 748962.

Also, please don't cc cq folks directly, we actually do triage our bug tracker :) 

Ack, thanks.

I've reverted the swarmbucket changes for the moment, and it looks like the CQ is working again.