Asp.Net Core; Raspberry Pi 4: Unable to load the service

I am trying to run my Asp.Net Core MVC Application on a Raspberry Pi 4. And the build error says:

/opt/dotnet/sdk/3.1.102/NuGet.targets(123,5): error : Unable to load the service index for source http://private-repository/SEM.NuGetServer.v3/feed/index.json. [/home/pi/Desktop/TestPlauder/PlaudertischSoftware/PlaudertischSoftware/PlaudertischSoftware.csproj]
/opt/dotnet/sdk/3.1.102/NuGet.targets(123,5): error :   Name or service not known [/home/pi/Desktop/TestPlauder/PlaudertischSoftware/PlaudertischSoftware/PlaudertischSoftware.csproj]

The build failed. Fix the build errors and run again.

And this is my nuget.config:

<?xml version="1.0" encoding="utf-8"?> 
<configuration>
  <packageSources>
    <add key="kbciot" value="http://private-repository/SEM.NuGetServer.v3/feed/index.json" protocolVersion="3" />
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
    <add key="private package source" value="https://nuget.devexpress.com/xtdT9FsWlktTtkCZagjJFdoTbs5PbjmRTqTWb586HWdsn2l4Ho/api" />
  </packageSources>
</configuration>      

I have no idea how to fix this.