One of the jobs tries to start but it does not:
In the 'All' view you don't see the job running.
On the slaves you don't see the job running.
As soon as you open the job, the next run is busy.
If we open the console output there is nothing.

When we open the time trendline of the job we see that it is executing on the master instead of the slave which it is tied to. See attachment.
The job is stuck and the only way to fix it is to restart Hudson.