BizTalk360 will try to connect to the underlying BizTalk environment using the IIS application pool identity credentials. If the user does not have enough rights to access the underlying environment, then you'll see this error.

Login failed for user "SERVERNAME\ApplicationPoolUser"

You can easily fix this issue by following the below procedure:

The account under which the BizTalk360 IIS application pool is running must be a memeber of the following groups

  • Local Administrators Group (in all servers) 
  • BizTalk Server Administrators Group 
  • BizTalk Application Users Group 
  • BizTalk Isolated Host Users Group 
  • SSO Administrators Group 
  • IIS_IUSRS group