You noted the rest API URL in branding. Do you have the VDC public url set as well?
I have seen this before if the 2 urls are not set correctly.
Also I have seen a bug before where if you did not have access to the cell (by what the cell thinks itself is, sometimes IP, sometimes FQDN) - the upload will fail, it wants to bypass the LB. Only way I know to see if you are seeing this issue is to get a RestAPI connection to the cloud, and get a vAppTemplate XML dump. At the top the "download" url should have the public address you gave, under some conditions I have seen it give the cell details (causing issues). If that is the case a support case might be needed.