Continuing the discussion from Best practise for buildout in CI:
we were using plone.app.async (and thus zc.async as well) and moved to collective.taskqueue (thanks @datakurre for it!), the only part that was missing when removing p.a.async stack was a dangling ERROR message logged on the console at every start up.
The solution is pretty quick and simple: https://gist.github.com/gforcada/0fd8c176559ae8773d09
You can put that on an upgrade step or run it as a script (i.e bin/instance run my_script.py).
I hope it helps!