My projects can't find each other in Visual Studio

I'm currently using version 20.20.2.2f1 and the newest version of Visual Studio Community 2019 (16.8.4)

In my project I'm using SimpleInput found in Asset Store. This has worked fine until yesterday when I tried to add a CustomEditor script. Now neither SimpleInput or my CustomEditor works

The problems are that CustomEditor (Assembly-CSharp-Editor project) can't find the classes from the main project (Assembly-CSharp) and the main project can't find SimpleInput (SimpleInput.Runtime project)

At first when I added my CustomEditor script, the SimpleInput was fine, but in search of a solution for CustomEditor I managed to break any connection to SimpleInput. This was done by adding Assembly Definition and Assembly Definition Reference objects to the main project and the editor folder. While this did fix the CustomEditor problem, it created the problem for CustomEditor. I tried to roll it back to before, but now nothing works. I tried reimporting SimpleInput

The Build Order seems fine. It has the following: SimpleInput.Runtime SimpleInput.Editor Assembly-CSharp Assembly-CSharp-Editor

The problem is only when it's open in visual Studio. I can still run it in Unity, but it makes scripting much more difficult

I tried to delete all .sln and .csproj files to get Unity to rebuild them, but that didn't help

SimpleInput.Editor requires SimpleInput.Runtime, but as with all others, this one can't find the required project. I can see under references that the reference seems to be correct. Here's what's written in the csproj files:

SimpleInput.Runtime.csproj:

 <PropertyGroup>
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
     <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
     <ProductVersion>10.0.20506</ProductVersion>
     <SchemaVersion>2.0</SchemaVersion>
     <RootNamespace></RootNamespace>
     **<ProjectGuid>{E2857400-636D-B3CC-C6EB-0802E8039CF4}</ProjectGuid>**
     <OutputType>Library</OutputType>
     <AppDesignerFolder>Properties</AppDesignerFolder>
     <AssemblyName>SimpleInput.Runtime</AssemblyName>
     <TargetFrameworkVersion>v4.7.1</TargetFrameworkVersion>
     <FileAlignment>512</FileAlignment>
     <BaseDirectory>.</BaseDirectory>
   </PropertyGroup>

Assembly-CSharp.csproj:

 <ItemGroup>
     <ProjectReference Include="SimpleInput.Runtime.csproj">
       **<Project>{E2857400-636D-B3CC-C6EB-0802E8039CF4}</Project>**
       <Name>SimpleInput.Runtime</Name>
       <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
     </ProjectReference>
     <ProjectReference Include="SimpleInput.Editor.csproj">
       <Project>{2054931F-D4C4-1192-5058-067B703F24F3}</Project>
       <Name>SimpleInput.Editor</Name>
       <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
     </ProjectReference>
   </ItemGroup>

The references are there, Visual Studio just can't seem to figure it out

Anybody else who has experienced this?