Initially I thought it was an issue with the new extension version, but I downgraded and the issue remains.
No source generators work with the following error:

Generator 'RegexGenerator' failed to initialize. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'FileNotFoundException' with message 'Could not load file or assembly 'System.Collections.Immutable, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.
'.
System.IO.FileNotFoundException: Could not load file or assembly 'System.Collections.Immutable, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.

File name: 'System.Collections.Immutable, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
   at System.Text.RegularExpressions.Generator.RegexGenerator.Initialize(IncrementalGeneratorInitializationContext context)
   at Microsoft.CodeAnalysis.GeneratorDriver.RunGeneratorsCore(Compilation compilation, DiagnosticBag diagnosticsBag, Func`2 generatorFilter, CancellationToken cancellationToken) in /_/src/Compilers/Core/Portable/SourceGeneration/GeneratorDriver.cs:line 252
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dotnet new console

then add this to Program.cs:

static partial class Regexes
{
    [GeneratedRegex(@"[\x00-\x1f\\/:\""*?<>\|]|\.+$")]
    public static partial Regex NotAllowedInFolderChars { get; }
}

dotnet new console (using .NET 9 + .NET 8 combined SDK)

then add this to Program.cs:

static partial class Regexes
{
    [GeneratedRegex(@"[\x00-\x1f\\/:\""*?<>\|]|\.+$")]
    public static partial Regex NotAllowedInFolderChars { get; }
}

dotnet new console (using .NET 9 + .NET 8 combined SDK, resulting in a net9.0 project)

then add this to Program.cs:

static partial class Regexes
{
    [GeneratedRegex(@"[\x00-\x1f\\/:\""*?<>\|]|\.+$")]
    public static partial Regex NotAllowedInFolderChars { get; }
}