My Jenkins box was showing me an error message about proxying being set up wrongly in its Management page. It turns out recent changes to Jenkins have either triggered this problem, or triggered the error message because of better monitoring (I couldn't actually figure out which). At the same time, the Stylesheets job, which checks out code from GitHub, was failing to trigger, or possibly failing to correctly poll the GitHub repo (again, I couldn't quite figure out which, although I think it was failing to poll), so clearly something was wrong. I consulted this page, and modified the /etc/apache2/sites_available/jenkins file to add "nocanon" to the first line shown below, and to add the second and third lines. After a reboot, Jenkins came up without the error message in the Management page, and the Stylesheets job instantly polled the repo, updated, and started building.
ProxyPass / http://localhost:8080/ nocanon ProxyPassReverse / http://localhost:8080 AllowEncodedSlashes NoDecode