Observed in a Jenkins server after restarting with some running builds. Since all Queue calls hanged, the system seemed to be frozen—no branch indexing, etc. Even taking a fresh support bundle hanged for the same reason (probably support-core ought to interrupt component threads which do not finish within some fixed timeout). Cannot cancel running builds for the same reason. The only way to recover was to disable the TCB plugin.
Have also heard a field report of a server running TCB 2.0 exhibiting similar symptoms, but have not yet confirmed duplicate, pending existence of a thread dump.