I am running a similar setup: Selenium 3.40, Chrome 61, chromedriver 2.33 running with xvfb on ubuntu 16.04.
I was getting the same Chrome error intermittently. It seems that sometimes, the chromedriver fails to cleanup the temp files associated with the Chrome profile.
A workaround for me is to cleanup the temp files before running tests:
rm -rf /tmp/.org.chromium.Chromium*I expect this will be resolved in future versions of chromedriver, but for now this solves the problem in my case.