I just pushed out a promotion process to ~400 builds after successfully testing with about a dozen. The first week after about 30 successful promotions, I ran in to this issue. In my case, the promotions all consist of a single "Send build artifacts over SSH" task. I have the same traceback as in the description above in my jenkins log. In the UI the promotion task is stuck (not cancellable) and shows no logs. Restarting Jenkins and re-executing the promotion was the only way to fix the situation. I wouldn't have even known about this issue if one of my users didn't wait around for the finish and attempt to re-execute the task (and hence queue up a visible task in the global queue). For now I need to check the jenkins log for these issues to stay on top of these. I'll report back on the frequency of successful promotions vs. stuck promotions after a few days.