Few things to take into account, i have seen a lot of reports about this.
My issues appears to be McAfee and Windows Defender battling it out. Do you happen to have both products running? try setting group policy to specifically deny Windows Defender from running.
Another problem could be users that have had their profiles generated via 1709 and then tried logging into a desktop that are 1809. i believe 1709 creates v2 profiles while 1809 generates v6?
Try having a user without a profile login to only a 1809 desktop that way they get a v6 profile and see if they experience the notorious "Black Screen"
Did you install the vRealize Operations for Horizon Desktop Agent? It is part of the agent install. I believe that includes the logon monitor so metrics can be uploaded to vrealize. Try uninstalling it.