There can be many reasons for this error. The 400 ERROR means a bad HTTP request. Please update the on-premise gateway to the latest version. If you already have updated the gateway to the latest version, then you can refer to the steps below for troubleshooting.
1. Check if the server is using a proxy and the proxy. Some proxies restrict traffic to only ports 80 and 443. By default, communication to Azure Service Bus will occur on ports other than 443.
2. You can force the gateway to communicate with Azure Service Bus using HTTPS instead of direct TCP. You will need to modify the Microsoft.PowerBI.DataMovement.Pipeline.GatewayCore.dll.config file. Change the value from AutoDetect to HTTPS. This file is located, by default, at C:\Program Files\On-premises data gateway.
3. Restart the gateway and check if the issue persists.
To stop the service, run this command:
''' net stop PBIEgwService '''
To start the service, run this command:
''' net start PBIEgwService '''
4. You may export the data gateway log by selecting Diagnostics and then selecting the Export logs link near the bottom of the page. Reference: https://docs.microsoft.com/en-us/power-bi/service-gateway-onprem-tshoot#tools-for-troubleshooting.