Error NU1105 Unable to find project information - The project file may be invalid or missing targets required for restore

All of a sudden, I am getting the following errors for 3 projects in a solution:

Error NU1105 Unable to find project information for 'C:\code\example\src\libs\example.I18n\example.I18n.csproj'. 
The project file may be invalid or missing targets required for restore.

The only thing that has changed in the project is a couple of DB changes, but I never had any issues in the past. The only other thing is that I updated to Visual Studio 2017 15.5. Could that cause issues?

I have tried removing and recloning the solution from source control, but still getting errors. No problems on my colleagues' machines, so it must be something local.

Example of one of the .csproj files if this helps:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net452</TargetFramework>
    <AssemblyName>Example.I18n</AssemblyName>
    <PackageId>Example.I18n</PackageId>
    <GenerateAssemblyConfigurationAttribute>false</GenerateAssemblyConfigurationAttribute>
    <GenerateAssemblyCompanyAttribute>false</GenerateAssemblyCompanyAttribute>
    <GenerateAssemblyProductAttribute>false</GenerateAssemblyProductAttribute>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Core" Version="1.1.2" />
    <PackageReference Include="Microsoft.Extensions.Logging" Version="1.1.1" />
    <PackageReference Include="MessageFormat" Version="1.0.1" />
  </ItemGroup>

  <ItemGroup Condition=" '$(TargetFramework)' == 'net452' ">
    <Reference Include="System" />
    <Reference Include="Microsoft.CSharp" />
  </ItemGroup>

</Project>

For me, the casing of the project file on disk did not match the casing in the solution file.

Say I had a solution with LibraryA.csproj and LibraryB.csproj, where LibraryB.csproj has a reference to LibraryA.csproj. Having an incorrect casing for LibraryA.csproj in the solution file would cause NU1105 when building LibraryB.csproj:

Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "LibraryA", "LibraryA\Librarya.csproj", "{24DEBB3B-762A-491D-8B83-6D078C0B30C0}"

I started seeing this problem after upgrading to version 15.5 of Visual Studio 2017. I did not encounter this problem with version 15.4.5.