Failed to execute action 'publish' System.Xml.XmlException: There are multiple root elements

Symptoms

wsus-publisher.log:
2020-09-03 10:15:46.0797|INFO|1|Parallels.Pma.WsusPublisher.Program|Current directory set to 'C:\Users\sperry1\AppData\Roaming\.pmm-aus-temp'
2020-09-03 10:15:48.4619|INFO|1|Parallels.Pma.WsusPublisher.Actions.GetSignCert|Package signing certificate saved as C:/Users/sperry1/AppData/Local/Temp\WSC8B24.tmp
2020-09-03 10:16:17.8027|INFO|1|Parallels.Pma.WsusPublisher.Program|Current directory set to 'C:/ProgramData/Parallels/Sup/WsusPublisher\.pmm-aus-temp'
2020-09-03 10:16:17.8965|FATAL|1|Parallels.Pma.WsusPublisher.Program|Failed to execute action 'publish'
System.Xml.XmlException: There are multiple root elements. Line 2, position 2.
   at System.Xml.XmlTextReaderImpl.Throw(Exception e)
   at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
   at System.Xml.XmlLoader.LoadNode(Boolean skipOverWhitespace)
   at System.Xml.XmlLoader.LoadDocSequence(XmlDocument parentDoc)
   at System.Xml.XmlDocument.Load(XmlReader reader)
   at System.Xml.XmlDocument.Load(String filename)
   at Parallels.Pma.WsusPublisher.Actions.UpdatePublisher.LoadPListFromFile(String path)
   at Parallels.Pma.WsusPublisher.Actions.UpdatePublisher.Run(Dictionary`2 arguments, FsStore fsStore)
   at Parallels.Pma.WsusPublisher.Program.Run(ProgramArgs args)

Cause

PMM SUP service failed to authenticate against the web proxy.

*.sucatalog files from C:\ProgramData\Parallels\Sup\Catalogs\ look like this:

Resolution

Please configure the web proxy to allow PMM SUP connecting to http://swscan.apple.com/*