Blazor project from standard template errors after edit

I'm using Visual Studio Community 2026, December 2025 Feature Update (18.1.1), which is the latest version right now.

I create a new project using the 'Blazor Web App' template. I use the default options:

I can run this base project just fine, everything works. I can stop and re-start it just fine.

If I now make any edits to the project, no matter how simple, i.e.

In the Counter.razor component I add a simple <p> tag just after the <h1>Counter</h1> tag:

@page "/counter"<PageTitle>Counter</PageTitle><h1>Counter</h1><p>Simple counter</p><p role="status">Current count: @currentCount</p><button class="btn btn-primary" @onclick="IncrementCount">Click me</button>@code {    private int currentCount = 0;    private void IncrementCount()    {        currentCount++;    }}

When I run it I get an 'Exception Unhandled dotnet.w4yrqk0izw.js' error in VS:

Call Stack:

I can press F5 to continue and the site loads, but any interaction doesn't work (i.e. clicking the 'Click Me' button doesn't increment the counter)

In the browser console window there are a couple of errors:

blazor.web.wmp8iwak4q.js:1 Failed to load module script: Expected a JavaScript-or-Wasm module script but the server responded with a MIME type of "". Strict MIME type checking is enforced for module scripts per HTML spec.

(anonymous) @ blazor.web.wmp8iwak4q.js:1

(anonymous) @ blazor.web.wmp8iwak4q.js:1

load @ blazor.web.wmp8iwak4q.js:1

(anonymous) @ blazor.web.wmp8iwak4q.js:1

await in (anonymous)

Kr @ blazor.web.wmp8iwak4q.js:1

startLoadingWebAssemblyIfNotStarted @ blazor.web.wmp8iwak4q.js:1

registerComponent @ blazor.web.wmp8iwak4q.js:1

(anonymous) @ blazor.web.wmp8iwak4q.js:1

Ki @ blazor.web.wmp8iwak4q.js:1

blazor.web.wmp8iwak4q.js:1 Uncaught (in promise) Error: Failed to start platform. Reason: TypeError: Failed to fetch dynamically imported module: https://blazorapp1.dev.localhost:7137/\_framework/dotnet.w4yrqk0izw.js

at Vr (blazor.web.wmp8iwak4q.js:1:176024)

Vr @ blazor.web.wmp8iwak4q.js:1

await in Vr

Jr @ blazor.web.wmp8iwak4q.js:1

startWebAssemblyIfNotStarted @ blazor.web.wmp8iwak4q.js:1

resolveRendererIdForDescriptor @ blazor.web.wmp8iwak4q.js:1

determinePendingOperation @ blazor.web.wmp8iwak4q.js:1

refreshRootComponents @ blazor.web.wmp8iwak4q.js:1

(anonymous) @ blazor.web.wmp8iwak4q.js:1

If I stop the project, and open the Powershell window and issue a 'dotnet build' command, then the project runs fine. Until I make any other edit and it errors again as before.

Literally no other changes have been made from the code generated by the template. Checking the configuration I see that both the main and client projects are set to build when run:

I have tried Chrome, Edge, and Firefox browsers. All the same (although Firefox doesn't trigger the error in VS, but does show the console error and doesn't work).

As this is basically the template project I'm reasonably sure that it's not me, although it may be something on my PC. But I have no idea what.

Anyone help please?