waitress is updated to version 2.1.1 to mitigate a vulnerability in that package. As waitress no longer supports Python versions less than 3.7 it is not advised to run Plone 5.2 on Python 2.7 or 3.6 any longer, even though they are still supported by Plone itself. You get an older waitress version then. If you must use an old Python version, please switch to a different WSGI server. See the recommendations in the Zope documentation.
WARNING: the new waitress 2.1.1 does seem to suffer from a possible race condition leading to the process quitting. If you are affected by this, you can downgrade to 2.1.0 (which has a known security vulnerability, as mentioned above) or use a different WSGI server.
plone.app.linkintegrity: Track link integrity of referenced PDFs and other site objects in IFRAME SRC references.
plone.outputfilters: Resolve UIDs in SRC attribute of of SOURCE and IFRAME elements.
plone.app.querystring: Add lazy attribute to vocabularies to prevent fetching any results.
plone.schema: Use indent in json.dumps to make JSON readable in the widget.
I cannot tell from this short error message if this is related to waitress.
It should be fine to use the newer version though. If that unexpectedly gives more problems, you can easily revert.
AttributeError: 'NoneType' object has no attribute 'sync'
exec(compile(__file__f.read(), __file__, "exec"))
File "/buildout-cache/eggs/Zope-4.8.1-py3.7.egg/Zope2/Startup/serve.py", line 255, in <module>
sys.exit(main() or 0)
File "/buildout-cache/eggs/Zope-4.8.1-py3.7.egg/Zope2/Startup/serve.py", line 251, in main
return command.run()
File "/buildout-cache/eggs/Zope-4.8.1-py3.7.egg/Zope2/Startup/serve.py", line 217, in run
serve()
File "/buildout-cache/eggs/Zope-4.8.1-py3.7.egg/Zope2/Startup/serve.py", line 203, in serve
server(app)
File "/buildout-cache/eggs/plone.recipe.zope2instance-6.11.0-py3.7.egg/plone/recipe/zope2instance/ctl.py", line 942, in serve
return serve_paste(app, global_conf, **kws)
File "/buildout-cache/eggs/plone.recipe.zope2instance-6.11.0-py3.7.egg/plone/recipe/zope2instance/ctl.py", line 917, in serve_paste
waitress.serve(app, **kw)
File "/buildout-cache/eggs/waitress-2.1.1-py3.7.egg/waitress/__init__.py", line 19, in serve
server.run()
File "/buildout-cache/eggs/waitress-2.1.1-py3.7.egg/waitress/server.py", line 325, in run
use_poll=self.adj.asyncore_use_poll,
File "/buildout-cache/eggs/waitress-2.1.1-py3.7.egg/waitress/wasyncore.py", line 245, in loop
poll_fun(timeout, map)
File "/buildout-cache/eggs/waitress-2.1.1-py3.7.egg/waitress/wasyncore.py", line 172, in poll
r, w, e = select.select(r, w, e, timeout)
OSError: [Errno 9] Bad file descriptor