Missing netstandard 2.0 reference in Xamarin.Forms project when building with Jenkins

I'm getting over 1500 errors like this, when I try to build an updated Xamarin.Forms project with Jenkins:

>App.xaml.cs(24,32): error CS0012: Der Typ "Object" ist in einer nicht referenzierten Assembly definiert. Fügen Sie einen Verweis auf die Assembly "netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" hinzu.

Translated error:

>CS0012: The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.

If I build the project with the same machine, but within Visual Studio 2017 it works without problems (as on my local machine). The build command for Jenkins looks like this

"C:\Program Files (x86)\Microsoft Visual Studio17\BuildTools\MSBuild.0\Bin\MSBuild" /p:ReferencePath=C:\jenkins\workspace\somelibrary\ SomeProject/SomeProject/SomeProject.csproj /t:Restore /p:Configuration=Release /t:Build 

Previously I updated to .NET Standard 2.0 and Xamarin.Forms 3.4.0.1039999. The build server itself is running on <s>Windows 10 1703</s>Windows 10 1909. <s>I have UWP in my Xamarin.Forms project, but I'm not really using it.</s> (I read that you need at least Windows 10 1709 if you want to use .NET Standard 2.0 together with UWP.)

The project is also referencing a NuGet package, which isn't compatible with .NET Standard 2.0 yet. Here I get the warning

>Warning NU1701 Package 'xxx' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETStandard,Version=v2.0'. This package may not be fully compatible with your project.

This could be a reason why there are some problems, because it is not fully compatible.

What I tried:

• clean build, deleted bin and obj folder
• use of <Reference Include="netstandard" /> without success at building with Jenkins. Reference to NETStandard.Library throws a warning in my local build environment (but it is installed)
• updated Visual Studio Build Tools 2017 and Visual Studio 2017 to the latest version (15.9.18)
• installed .NET Core SDK, .NET Core Build Tools, .NET Framework 4.7.2 SDK on the build server
• extended PATH environment variable with C:\Program Files\dotnet, set global environment variable MSBuildSDKsPath to C:\Program Files\dotnet\sdk\2.1.509\Sdks
• deleted UWP project (still exists on folder, but not in *.sln)
• made some research after similar problems from other users, but didn't find something that solved the issue

What I observed:

• building the project works fine with Visual Studio 2017 on the build server with Windows 10 1703
• building the project works fine with Visual Studio 2017 on my local machine with Windows 10 1809
• building the project works fine with msbuild and command line on the build server (NuGet packages references correctly netstandard2.0 instead of netstandard1.3 and so on in the NuGetFallbackFolder)

What I could try:

• <s>update the build server to Windows 10 1709 or higher</s>
• replace/update the library, which still uses .NET 4.6.1
• upgrade to Visual Studio 2019
• create a .NET Standard project from scratch and copy all files over (and loosing GIT history)

But what I don't get is that it is working fine in Visual Studio but not with msbuild/Jenkins. Seems that a reference is missing or something is not installed.

What should I do?

Edit:

Extract of the .csproj from the Xamarin.Forms part of the project

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <NeutralLanguage>en</NeutralLanguage>
    <AssemblySearchPaths>
        $(AssemblySearchPaths);
        $(ReferencePath);
    </AssemblySearchPaths>
  </PropertyGroup>
  <ItemGroup>
    <None Remove="picture.png" />
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="Plugin" Version="5.2.0" />
  </ItemGroup>

  <ItemGroup>
    <Reference Include="SomeLibrary">
      <HintPath>pat\to\library\netstandard2.0\SomeLibrary.dll</HintPath>
    </Reference>
  </ItemGroup>

    <ItemGroup>
        <Compile Update="Common\Localization\AppResources.Designer.cs">
            <AutoGen>True</AutoGen>
            <DesignTime>True</DesignTime>
            <DependentUpon>AppResources.resx</DependentUpon>
        </Compile>
    </ItemGroup>
    <ItemGroup>
        <EmbeddedResource Update="Common\Localization\AppResources.de.resx">
            <SubType>Designer</SubType>
        </EmbeddedResource>
        <EmbeddedResource Update="Common\Localization\AppResources.resx">
            <Generator>PublicResXFileCodeGenerator</Generator>
            <LastGenOutput>AppResources.Designer.cs</LastGenOutput>
            <SubType>Designer</SubType>
        </EmbeddedResource>
    </ItemGroup>
    <ItemGroup>
      <None Update="App.xaml">
        <Generator>MSBuild:Compile</Generator>
      </None>
      <None Update="SomePage.xaml">
        <Generator>MSBuild:Compile</Generator>
      </None>
    </ItemGroup>

</Project>

I think I solved it with this

"C:\Program Files (x86)\Microsoft Visual Studio17\BuildTools\MSBuild.0\Bin\MSBuild" SomeProject/SomeProject/SomeProject.csproj /t:Restore /p:Configuration=Release

"C:\Program Files (x86)\Microsoft Visual Studio17\BuildTools\MSBuild.0\Bin\MSBuild" /p:ReferencePath=C:\jenkins\workspace\somelibrary\ SomeProject/SomeProject/SomeProject.csproj /p:Configuration=Release /t:Build

So the solution is to make two calls: one for restore and one for build. I do this for the Xamarin.Forms project and platform specific project separately.