Failed to start platform. Reason: [object ProgressEvent]

[GerritBergen]

Describe the bug

Often times when I start my client side blazor app the browser console throws Failed to start platform. Reason: [object ProgressEvent] and the app never loads. Just stays on the splash screen.

To Reproduce

I wish I could confidently reproduce this but it just happens sometimes, and a few browser refreshes makes it work again.

Screenshots

Additional context

Using FireFox 69.0b3

.NET Core SDK (reflecting any global.json):
Version: 3.0.100-preview6-012264
Commit: be3f0c1a03

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18362
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.0.100-preview6-012264\

Host (useful for support):
Version: 3.0.0-preview6-27804-01
Commit: fdf81c6faf

.NET Core SDKs installed:
2.1.700 [C:\Program Files\dotnet\sdk]
2.1.800-preview-009696 [C:\Program Files\dotnet\sdk]
3.0.100-preview6-012264 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.0-preview6.19307.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.0-preview6-27804-01 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.0.0-preview6-27804-01 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

[mkArtakMSFT]

Thanks for contacting us, @GerritBergen.
It would help, if you could provide some more details and/or logs, if you have any.

[mkArtakMSFT]

Also, do you see any failed requests in the Network tab?

[GerritBergen]

Running in Edge Dev gives a bit more info. Most of the dll files are returning 304.
As of this run, Mono.WebAssembly.Interop.Dll & Microsoft.AspNetCore.Components.Browser.Dll shows (cancelled) in the status column. But that's just the weird thing, this changes with each refresh. Sometimes they all come in. Other times more/different ones show cancelled.

I'm sure I've got something setup wrong somewhere. I forgot to mention in the original post that I am running using Kestral if that helps.

EDIT: I've rerun using IIS (no kestral) and it still happens

[GerritBergen]

Probably useless information because it's minified JS, but this is where the error occurs in blazor.webassembly.js:

[mkArtakMSFT]

@SteveSandersonMS do you have any recommendations regarding this?

[SteveSandersonMS]

It definitely appears to be a problem with serving files to the browser, not a problem with any code running inside the browser. This is separate from Blazor I'm afraid.

You'll need to figure out why your server is unable to send static files to your browser.

[GerritBergen]

Do you have any suggestions as to what could cause that?

[SteveSandersonMS]

It could be that your web server is faulty or misconfigured in some way, your browser is faulty or misconfigured in some way, or that some other thing such as anti malware software is interfering. I’m afraid that’s all I can suggest.

[GerritBergen]

A bunch of debugging later, I found this error in my server console right when the browser fails.

Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.Http2Connection ErrorException: HTTP/2 connection error (PROTOCOL_ERROR): Pseudo-header field found in request headers after regular header fields.

Not really sure what's going on, but disabling Http2 made the issue go away for me.
Not ideal, but I think I'll stick with that for now.

Thanks for your help