Will not start builds even though there are available slots on executor

Sometimes our nodes won't be able to start new builds even though there are free slots available.

A workaround for the slaves is to disconnect/connect the slave and it will start to schedule builds again.

I have observed that when this happens for a slave the slave has fewer threads ongoing than an idle slave.

Attaching thread dumps when this happens and after doing an disconnect/connect.

We have seen this issue both on Windows(jlnp) slaves and linux(ssh) slaves as well as on the master node which is running linux.