"No input sources specified"

I'm upgrading a client's old VB.Net (2003) desktop app, using VS 2019 Community. I've one issue that I cannot resolve. I'm getting a BC2008 error code, with description "no input sources specified".

I've done some googling, and some have found a re-install of VS has somehow done the trick, but hasn't worked for me. I have no idea how to even debug this one. As far as I can tell, the project is the same as the 2003 version before-upgrade.

Any ideas?

Error:

The DataLayer project:

Edit:

Contents of the .vbproj file:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" DefaultTargets="Build">
  <PropertyGroup>
    <ProjectType>Local</ProjectType>
    <ProductVersion>7.10.3077</ProductVersion>
    <SchemaVersion>2.0</SchemaVersion>
    <ProjectGuid>{4934644F-F06D-4F6D-B0D1-BF892686E073}</ProjectGuid>
    <SccProjectName>
    </SccProjectName>
    <SccLocalPath>
    </SccLocalPath>
    <SccAuxPath>
    </SccAuxPath>
    <SccProvider>
    </SccProvider>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ApplicationIcon />
    <AssemblyKeyContainerName />
    <AssemblyName>DataLayer</AssemblyName>
    <AssemblyOriginatorKeyFile />
    <AssemblyOriginatorKeyMode>None</AssemblyOriginatorKeyMode>
    <DefaultClientScript>JScript</DefaultClientScript>
    <DefaultHTMLPageLayout>Grid</DefaultHTMLPageLayout>
    <DefaultTargetSchema>IE50</DefaultTargetSchema>
    <DelaySign>false</DelaySign>
    <OutputType>Library</OutputType>
    <OptionCompare>Binary</OptionCompare>
    <OptionExplicit>On</OptionExplicit>
    <OptionStrict>Off</OptionStrict>
    <RootNamespace>DataLayer</RootNamespace>
    <StartupObject>DataLayer.%28None%29</StartupObject>
    <FileUpgradeFlags>
    </FileUpgradeFlags>
    <MyType>Windows</MyType>
    <UpgradeBackupLocation>
    </UpgradeBackupLocation>
    <OldToolsVersion>0.0</OldToolsVersion>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <OutputPath>bin\</OutputPath>
    <DocumentationFile>DataLayer.xml</DocumentationFile>
    <BaseAddress>285212672</BaseAddress>
    <ConfigurationOverrideFile />
    <DefineConstants />
    <DefineDebug>true</DefineDebug>
    <DefineTrace>true</DefineTrace>
    <DebugSymbols>true</DebugSymbols>
    <Optimize>false</Optimize>
    <RegisterForComInterop>false</RegisterForComInterop>
    <RemoveIntegerChecks>false</RemoveIntegerChecks>
    <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
    <WarningLevel>1</WarningLevel>
    <NoWarn>42016,42017,42018,42019,42032,42353,42354,42355</NoWarn>
    <DebugType>full</DebugType>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <OutputPath>bin\</OutputPath>
    <DocumentationFile>DataLayer.xml</DocumentationFile>
    <BaseAddress>285212672</BaseAddress>
    <ConfigurationOverrideFile />
    <DefineConstants />
    <DefineDebug>false</DefineDebug>
    <DefineTrace>true</DefineTrace>
    <DebugSymbols>false</DebugSymbols>
    <Optimize>true</Optimize>
    <RegisterForComInterop>false</RegisterForComInterop>
    <RemoveIntegerChecks>false</RemoveIntegerChecks>
    <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
    <WarningLevel>1</WarningLevel>
    <NoWarn>42016,42017,42018,42019,42032,42353,42354,42355</NoWarn>
    <DebugType>none</DebugType>
  </PropertyGroup>
  <ItemGroup>
    <Import Include="Microsoft.VisualBasic" />
    <Import Include="System" />
  </ItemGroup>
  <ItemGroup>
    <Content Include="MySql.Data.dll" />
    <Content Include="Paperbase.dll" />
    <Content Include="SD.LLBLGen.Pro.DQE.SqlServer.NET11.dll" />
    <Content Include="SD.LLBLGen.Pro.ORMSupportClasses.NET11.dll" />
  </ItemGroup>
  <ItemGroup>
    <Folder Include="My Project\" />
  </ItemGroup>
  <Import Project="$(MSBuildToolsPath)\Microsoft.VisualBasic.targets" />
  <PropertyGroup>
    <PreBuildEvent />
    <PostBuildEvent />
  </PropertyGroup>
</Project>

It's about the missing source code file. Try to see if all the source code files are available and loaded. The below article from Microsoft says the same thing

https://learn.microsoft.com/en-us/dotnet/visual-basic/misc/bc2008