You received a red banner limiting you to 1000 tasks.
Release : ALL Versions
1000 is the default "out-of-the-box" limitation.
This is working as designed, only 1000 active tasks can be running. The agents assigned tasks need to execute the tasks or time out before more can be assigned. The default timeout is 15 minutes.
In the event where the agent needs to be shutdown in mass then it is recommended to utilize software delivery like (SCCM) and the service_shutdown agent tool.
If agents need to be disabled (not shutdown) in mass then it is recommended to use an agent configuration that monitors no channels and default action is to ignore instead.