NeoLoad 6.0.0 Free version can only have two load generators?

I installed two additional Load Generators (6.0.0) from Docker. However, except localhost, in Controller I can only add one Load Generator successfully (show as green).

When I add the 2nd one, it just tries to connect, and then nothing happens.

If I remove the first one, add the 2nd one again, it will successfully connected. Meaning the Load Generator servers are working fine. It just can't be added into Controller- Runtime - Scenario.

The funny thing is, I can add the 2nd one when it is down (see screenshot, show as red). But then it can't be used.

Is this a limitation of free version?

Controller OS: Ubuntu Desktop 16.04 LTS
Load Generator: Docker image latest (6.0.0)
Controller: 6.0.0

Thanks.