I had a customer report a widget (plugin) installation issue to me. They had clients being unable to install a widget (and hence a plugin) from an update site on our servers (external to their network). The error was reported as below:
Unable to access "http://<host>/site.xml". Contains: Error parsing site stream. [White spaces are required between publicId and systemId.]
The issue turned out to be caused by a firewall issue and hence wrongly reported in the log. The issue was solved by relocating the update site to the customers network.