If you use the in-built vRealize Orchestrator instance shipped with the vRealize Automation appliance then you might run into this issue when working with the REST client:
Connection pool shut down (Workflow:Get-IdentityToken / Scripting (item3)#14)
The vRA appliance version I have (6.2 – note to self, need to update lab!) includes the plugin version 1.0.4 for REST. According to the release notes, this was fixed in 1.0.5 – typical!
So the solution is to upgrade the REST API plugin 🙂