Defender not updating - ValidateMapsConnection failed to establish a connection to MAPS

Microsoft Defender is not updating. When I click "Check for updates" in the window "Windows Security / Virus & threat protection" under "Virus & threat protection updates",  Defender searches for updates forever but also immediately shows me the message "Security intelligence is up to date.". Problem is that's not true an this message appears even if my definitions ar weeks old. 

I ran 

MpCmdRun.exe -ValidateMapsConnection

as documented on https://learn.microsoft.com/de-de/microsoft-365/security/defender-endpoint/command-line-arguments-microsoft-defender-antivirus?view=o365-worldwide and got the following error:

 ValidateMapsConnection failed to establish a connection to MAPS (hr=80070057 httpcode=451) CmdTool: Failed with hr = 0x80070057. Check C:\Users\<USERNAME>\AppData\Local\Temp\MpCmdRun.log for more information

the logfile looks like this:

MpEnsureProcessMitigationPolicy: hr = 0x1 ValidateMapsConnection ValidateMapsConnection failed to establish a connection to MAPS (hr=80070057 httpcode=451) MpCmdRun.exe: hr = 0x80070057.

I already tried the following steps without success:

• manual update with files from https://www.microsoft.com/en-us/wdsi/definitions/ (manual update worked but didn't solve the problem)
• trigger update in command line
• reset Defender with "MpCmdRun.exe -RemoveDefinitions -All" and restart 
• sfc/scannow
• firewall deactivated
• Resetting Windows Updates Components (Method 6 from https://appuals.com/definition-update-for-windows-defender-fails-with-error-0x80070643/)
• ...other things

  I don't have a 3rd party AV.

Any solutions? Is there a way I can do a ping to find out if I can connect to the update-server?