I am generating/changing lots of regression test jobs via groovy to
keep our build machines chewing during the night. Yesterday I had to
change some parameters in our jobs but the existing cron section
stayed the same. When diffing the config file it shows my changes, the
trigger section stayed the same. I did not restart Jenkins yesterday
evening and found that no job got scheduled during the night.

This is the error I got:
Aug 09, 2012 9:00:07 PM hudson.triggers.Trigger checkTriggers
WARNING: hudson.triggers.TimerTrigger.run() failed for XYZ_Tests
java.lang.NullPointerException
at hudson.triggers.TimerTrigger.run(TimerTrigger.java:53)
at hudson.triggers.Trigger.checkTriggers(Trigger.java:259)
at hudson.triggers.Trigger$Cron.doRun(Trigger.java:207)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)

When I re-edit the file by hand via Web UI and save, it works, but this is not an option for about 150 jobs. Restarting Jenkins after modifying the job via groovy script also helps.

The generated config file section of the jobs looks:
<triggers class="vector">
<hudson.triggers.TimerTrigger>
<spec>30 20 * * 1-5</spec>
</hudson.triggers.TimerTrigger>
</triggers>