I noticed today a number of jobs stuck in Jenkins. They were waiting for a slave machine to automatically start. However, it was not. It was indicating that the slave was starting but there was nothing in the logs. A manual restart had no effect. I decided to put Jenkins in shutdown mode, stop all running jobs, flush the queues and stop all build slaves. Once there was absolutely no activity on Jenkins I tried to manually start the stuck slave. However, nothing happened again. It seems like Jenkins is hung in the build start phase. I attach my list of plugins and thread dump.

Can anyone confirm Jenkins is hung based on the thread dump?
Any known workarounds?