The old 4.1.0 style installer allowed a single digit site id or a string
Due to the change to permit more complex numbers encapsulating virtual directories you now end up with a x.y.z form.
agentadmin --s 5 ...
Installation works normally
Unknown site id 5
The installer should assume that 5 means 5.1.1 if there is only one virtual directory and application, and the error should be more explicit if there is duplication. Troubleshooting steps should also be put in the docs.
This would affect most customers who script their installs using powershell.