As discussed in https://wiki.jenkins-ci.org/display/JENKINS/Promoted+Builds+Plugin?focusedCommentId=56459274#comment-56459274,

What is "Pending promotion (promotion not queued. Please re-execute promotion)" ?

I have set up a job, with Manually Approved promotion, that sends an Editable Email Notification and builds another project upon Promotion. I then did a few builds. When I go to the /server:8080/job/my-job/42/promotion/? page and click the Approve button, I see the message "Pending promotion (promotion queued)". When I refresh the page, then I see "Pending promotion (promotion not queued. Please re-execute promotion)". The email never gets sent and the other job does not get kicked off. I don't see any errors in the system log. Any ideas ?

I've got a similar problem.  I've downgraded to 1.11 for the time being, but the logs for the promotion are typically in the main hudson directory under jobs%JobName%\promotions%promotionName%\builds%BuildTimestamp%\

For this error the only thing I'm seeing in the logs is:

Legacy code started this job.  No cause information is available
FATAL: null
java.lang.NullPointerException
    at hudson.plugins.promoted_builds.Promotion.getUrl(Promotion.java:61)
    at hudson.model.Run.getEnvironment(Run.java:1771)
    at hudson.model.AbstractBuild.getEnvironment(AbstractBuild.java:725)
    at hudson.plugins.promoted_builds.Promotion.getEnvironment(Promotion.java:74)
    at hudson.plugins.promoted_builds.Promotion$RunnerImpl.decideWorkspace(Promotion.java:107)
    at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:409)
    at hudson.model.Run.run(Run.java:1362)
    at hudson.plugins.promoted_builds.Promotion.run(Promotion.java:93)
    at hudson.model.ResourceController.execute(ResourceController.java:88)
    at hudson.model.Executor.run(Executor.java:145)

Though there is a long line of characters before the null pointer exception.  As far as I can see, it looks like one of the targets where it creates the URL is failing, but I'm really not sure.  I'm trying to play around with it a bit more before putting in the ticket, but if anyone has more info, please feel free to share.