When adding an On-Prem or Hybrid organisation to Veeam Backup for Office 365 we get the following error:
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The On-Premises Exchange server does not have Powerhsell authentication enabled. 


On the On-Premises Exchange server, you need to enable Basic Authentication (See Below)
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  1. Open the Exchange Admin Center and Click on Servers
  2. Choose the Virtual Directories tab
  3. Click on Powershell (Default Web Site) or Powershell (Exchange Web Services web site)
  4. Choose authentication
  5. Enable “Basic Authentication”