You should not be able to add PowerShell host unless it is valid and you are able to connect to it.
At the time when PowerShell host was registered in vCO you were able to connect. Probably something has changed in the environment. You can try to add again the same host.
I guess you already gone trough them but here are some usefull link:
vCO PowerShell plug-in | VMware vCenter Orchestrator Blog - VMware Blogs
http://kaloferov.com/blog/using-credssp-with-the-vco-powershell-plugin/
http://kaloferov.com/blog/tag/powershell-host/
First thing I would do is to veerify that you are able to connect to winrm server and execute powershell scripts.
Verify that I am able to connect and authenticate against winrm service
>winrm identify -r:http://winrm_server:5985 -auth:kerberos -u:someuser@somedomain.com -p:password -encoding:utf-8
Verify that I am able to execute ps using winrs client
>winrs -r:http://winrm_server:5985 -u:someuser@somedomain.com -p:somepassword powershell.exe write-output 'Hello'
Once both command succeed will mean that winrm is properly configured and you can try registering power shell host in vCO.
One more thing. Do you use "Shared session" or "Session per user" mode for configured PowerShell host (see screenshot)?