Had an interesting one the other day, after configuring online backups to Azure DPM stopped working, having removed and reinstalled and reregistered the server I was getting a 100050 proxy error when trying to configure the backup vaults but we do not use a proxy (we used to).
After a bit of a search it seems that there was some left over proxy settings buried deep in the settings of IE but only visible if run as the system account! The agent uses the System account details if proxy settings are not set in the Register UI wizard. Here’s how to change this:
Run from an admin command prompt:
psexec -i -s “C:Program FilesInternet Exploreriexplore.exe”
Go to Tools -> Internet Options -> Connections -> LAN settings
Verify / change / Remove proxy settings for System account
Close Internet Explorer
After that the vault added successfully.
Hope it helps!