Failing to upload .snupkg file: The checksum does not match for the dll(s) and corresponding pdb(s)

I have a C# WPF library on NuGet. After publishing a new release (.nupkg file), I wanted to also upload the Symbols package (.snupkg file). But I get The checksum does not match for the dll(s) and corresponding pdb(s). error:

I have successfully published .snupkg file together with .nupkg file for 2 other C# .NET Framework libraries, following the exact same procedure as here. The only difference that I can think of is that this a WPF library (it has *.xaml files and it has a reference to PresentationFramework), whereas the other two aren't.

The procedure:

1. The settings for Release configuration in the .csproj files:

<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU'"><DebugType>portable</DebugType><Optimize>true</Optimize><OutputPath>bin\Release\</OutputPath><DefineConstants>TRACE</DefineConstants><ErrorReport>prompt</ErrorReport><WarningLevel>4</WarningLevel><DocumentationFile>bin\Release\GM.WPF.xml</DocumentationFile><DebugSymbols>true</DebugSymbols></PropertyGroup>

1. Rebuild (this generates .dll, .dll.config, .pdb, .xml files in bin/Release).
2. I have a .nuspec file with the metadata for the NuGet package in the same folder as the .csproj file (I don't think it's necessary to show the .nuspec file - not relevant to this problem).
3. Open PowerShell in this folder.
4. Execute command nuget pack -Properties Configuration=Release -Symbols -SymbolPackageFormat snupkg (this generates .nupkg and .snupkg files).
5. Upload .nupkg file to NuGet.
6. Upload .snupkg file to NuGet - this is where I get the error.

Please, if you need any other information, I can provide it.