I had the same experience yesterday so I looked over the master.cfg to determine why I was unable to cancel the build.
The current kill routines (canStopBuild(), can CancelPendingBuild()) use two items: The owner of the build (who initiated it) and the user logged into the Builder's web interface who is attempting to kill the build.
If the name of the user who initiated a force build and the name of the web user who is trying to kill a build match in the first few letters, then the build will be terminated. This worked in the pre-September master.cfg because the periodic scheduler was initiated by a pseudo-user (everyman) via a browser API interface.
However, the new Nightly scheduler used does not have an associated owner name so that test will always fail – and in this case – with an exception.
The kill functions could check for the use of Nightly Scheduler and just let anyone kill those presecheduled runs. You still want to check that a user invoked run can only be killed by the same user.
Regarding the error message, Russell is right---it just takes an exception and exits. This error occurs because BuildBot assumes that 'buildInfo["owner"]' exists in the Build object – but it doesn't for the Nightly scheduler. The kill routines need to ensure they don't try to access a non-existent field.