For future generations, to solve this issue, you need to change P4 setting server.global.client.views to 1. Default value is 0 but in this case the p4 client -o will not print output horde is expecting.
In my case, i modified horde source code. In ClientRecord.cs there is View property, that is serializable but not optional. So i made it optional, because i have no permission to change P4 setting above.