With version 6.4.0, if I do add host, I get a credentials error from the api.
I tried cmk version 6.3.0 instead: version 6.3.0 did not result in an API error.
I tried version 6.3.0 because there seems to be a change after 6.3.0 in how get/post requests are made if a password param is included. I hope this helps.