I ran into this small issue while trying to update a new VMware environment. First thing that you will get when searching for this issue is the VMware KB. Since this was an isolated environment, I checked to make sure that port 9084 is opened and connection works. Connection to vCSA on that port was working. so the problem was from somewhere else. While checking the network setup on the ESXi hosts, I noticed that they had configured the wrong DNS IP and after I changed it to the correct one, updates worked.
Host cannot download files from VMware vSphere Update Manager patch store