"Form field target_milestone was not defined; this may indicate a bug in your
If you are not logged in, attempt to submit changes to Bugzilla and login when
asked, it fails with the above error message. You can also reproduce this by
opening several Bugzilla reports, logging in on one of those pages, then
attempting to submit a comment in one of the pages seen before logging in. This
was working in a previous version of Bugzilla.
Should be fixed now. Please reopen if not the case.