The commit comment form doesn't always appear when saving a job configuration. The fault seems to lie in the PageMatcher regexp.
I've tested the current regexp against a couple of different URIs:
Some of the failed URIs are real-life examples of URIs where the commit comment form doesn't appear, others are more hypothetical.
I can work around the problem by adding an extra slash to my URIs when I configure jobs. Instead of opening view/All/job/myjob/configure in my browser, I open view/All//job/myjob/configure.