Problem with compiling my solution with .Net5 as the framework target

Hello.

I'm trying to compile my solution after I have updated all projects in the solution to .Net5. 

For my C++ projects I get always the error message

Microsoft.NuGet.targets(198, 5): Your project does not reference ".NETCoreApp,Version=v5.0" framework. Add a reference to ".NETCoreApp,Version=v5.0" in the "TargetFrameworks" property of your project file and then re-run NuGet restore.

My project file contains the correct target framework. When I open and compile the solution with Visual Studio I get no error, everything works fine.

Here is a part of my project file

<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <ItemGroup Label="ProjectConfigurations">
    <ProjectConfiguration Include="Debug|Win32">
      <Configuration>Debug</Configuration>
      <Platform>Win32</Platform>
    </ProjectConfiguration>
    <ProjectConfiguration Include="Release|Win32">
      <Configuration>Release</Configuration>
      <Platform>Win32</Platform>
    </ProjectConfiguration>
    <ProjectConfiguration Include="Debug|x64">
      <Configuration>Debug</Configuration>
      <Platform>x64</Platform>
    </ProjectConfiguration>
    <ProjectConfiguration Include="Release|x64">
      <Configuration>Release</Configuration>
      <Platform>x64</Platform>
    </ProjectConfiguration>
  </ItemGroup>
  <PropertyGroup Label="Globals">
    <VCProjectVersion>16.0</VCProjectVersion>
    <ProjectGuid>{D2F0AFB1-A1A1-4648-8389-CA36AD689563}</ProjectGuid>
    <TargetFrameworkVersion>v5.0</TargetFrameworkVersion>
    <Keyword>ManagedCProj</Keyword>
    <RootNamespace>LicenseDllWrapper</RootNamespace>
    <WindowsTargetPlatformVersion>10.0</WindowsTargetPlatformVersion>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>
  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">
    <ConfigurationType>DynamicLibrary</ConfigurationType>
    <UseDebugLibraries>true</UseDebugLibraries>
    <PlatformToolset>v142</PlatformToolset>
    <CLRSupport>NetCore</CLRSupport>
    <CharacterSet>Unicode</CharacterSet>
  </PropertyGroup>

I tried some different values for the TargetFramework as net5.0 or net5.0-windows or net5.0-windows7.0, but always the
same error.

How can I fix this problem? 
This problem prevents me from using Rider.

Best regards
Jörg

P.S.
After some more investigations I found something. Rider generates
a different projects.assets.json file then Visual Studio.

Here the generated file from VSS:
{
  "version": 3,
  "targets": {
    "net5.0-windows7.0": {
      "LicenseDllWrapper.Core/1.0.0": {
        "type": "project",
        "compile": {
          "bin/placeholder/LicenseDllWrapper.Core.dll": {}
        },
        "runtime": {
          "bin/placeholder/LicenseDllWrapper.Core.dll": {}
        }
      },
      "LicenseWrapperData/1.0.0": {
        "type": "project",
        "framework": ".NETCoreApp,Version=v5.0",
        "compile": {
          "bin/placeholder/LicenseWrapperData.dll": {}
        },
        "runtime": {
          "bin/placeholder/LicenseWrapperData.dll": {}
        }
      }
    }
  },
  
  And here the generated file from Rider:
  {
  "version": 3,
  "targets": {
    "net5.0-windows7.0": {
      "LicenseDllWrapper.Core/1.0.0": {
        "type": "project",
        "framework": "Unsupported,Version=v0.0",
        "compile": {
          "bin/placeholder/LicenseDllWrapper.Core.dll": {}
        },
        "runtime": {
          "bin/placeholder/LicenseDllWrapper.Core.dll": {}
        }
      },
      "LicenseWrapperData/1.0.0": {
        "type": "project",
        "framework": ".NETCoreApp,Version=v5.0",
        "compile": {
          "bin/placeholder/LicenseWrapperData.dll": {}
        },
        "runtime": {
          "bin/placeholder/LicenseWrapperData.dll": {}
        }
      }
    }
  },

I hope that helps a little bit.