Error when saving report to Power BI Report Server

Hi All

we are new to PowerBI server and we just installed March 2018 version.
Currently we are facing the same issue with publishing reports to the server using March 2018 Desktop. We configured server with local domain service account and SQL enterprise 2017 Software Assurance.

1. When trying to publish report from Desktop using service account we can select the server and see the folder however we are not able to publish the reports to the selected folder. When using any other user (defined on the portal to have access to the folders) we get error "Unexpected error encountered"

2. When trying to upload PBIX files (even empty) from the portal with both service account and other user, we get error "There was an error uploading your .pbix file. Verify that the file has not been corrupted and that the file extension matches the format of the file."

PBIT files are uploaded fine.

Analysis services are not configured. Is it a must for the PowerBI server?
Can the reason be that the user doesn’t have the pro license (to be able to upload) ?
Do we need to set NT service \PowerBI ... account as service account?

In RSPortal xxx log file I can see the error
|ERROR|35|The remote server returned an error: (407) Proxy Authentication Required.| RequestID = s_5f8d70f8-c1a9-4c0d-bcaa-7c9623f00c1a
we have local proxy but it should not be an issue.

Any advice is more than welcomed.