AggregateException Error in Blazor when deploying to Windows Server 2022 IIS 10

[doanhquang2005]

Issue Description:
When deploying a Blazor project using .NET 9 (Auto Render) to a Windows Server 2022 IIS 10, I encountered the following error:

blazor.web.js:1 ManagedError: AggregateException_ctor_DefaultMessage (ExpectedJsonTokens LineNumber: 0 | BytePositionInLine: 0.)
    at sn (dotnet.runtime.o8gq1i8bk6.js:3:26894)
    at Kt.resolve_or_reject (dotnet.runtime.o8gq1i8bk6.js:3:26449)
    at dotnet.runtime.o8gq1i8bk6.js:3:172526
    at dotnet.runtime.o8gq1i8bk6.js:3:172590
    at fr (dotnet.runtime.o8gq1i8bk6.js:3:35046)
    at Fc (dotnet.runtime.o8gq1i8bk6.js:3:172173)
    at 00b59d1a:0x1f0b5
    at 00b59d1a:0x1c7c3
    at 00b59d1a:0xea10

Environment:

• Framework: .NET 9 (Blazor Auto Render)
• IDE: Visual Studio 2022 v17.12.3
• Operating System: Windows Server 2022 IIS 10
• Note: This error does not occur when running the application locally on localhost.

Steps to Reproduce:

1. Build the Blazor project using .NET 9 in Visual Studio 2022.
2. Deploy the application to Windows Server 2022 IIS 10.
3. Open the application in a browser.

Expected Behavior:
The Blazor application should run without any errors.

Actual Behavior:
The application fails to load properly and throws the error mentioned above.

Questions:

1. Could this be related to IIS configuration on the server?
2. Are there specific deployment settings required for Blazor on a server environment?
3. How can this error be resolved?

Thank You!
Any help or guidance would be greatly appreciated.

[WolfgangHG]

We had the same error message, and this was the only match that was found by an internet search.

Update: Well, sometimes it helps to tell it to a rubber duck - after having done so, I had the idea to look at one or two other things, and suddenly I found that the reason was a file exclusion in ther process of copying the result of "dotnet publish" to another location. Though it was all an error on our side, I will keep this post - maybe it helps someone else who has the same problem for other reasons

Analysis 1:
Firefox reported that the error occured at char 157830 in blazor.web.json. This is this piece of code:

try{await cr.runMain(cr.getConfig().mainAssemblyName,[])}catch(e){console.error(e),Lo()}

This does not help further, it is just the startup code of the Blazor WebAssembly.

Analysis 2:
Attaching dnSpy to w3wp.exe reports a FileNotFoundException for a file "appsettings.json.br" (brotli compressed file)

Analysis 3:
Firefox network console reported that when loading the failing blazor page, the last request was a GET for a file https://myserver/MyWebApp/appsettings.json, which returns "200 OK" but a size of 0 bytes.

This "appsettings.json" file is not the one found in the root of the web app, but the one in the subdir "C:\inetpub\wwwroot\MyWebApp\wwwroot". Here and in its subdirs, several js or css files existed with ".br" and ".gz" alternatives, but no "appsettings.json.br".

Workaround 1:
In Firefox, open "about:config" and find the key network.http.accept-encoding.secure (default value: gzip, deflate, br, zstd). Change it to e.g. deflate and reload the page with CTRL+F5 => now it should work.

Workaround 2:
Pick the file "C:\inetpub\wwwroot\MyWebApp\wwwroot\appsettings.json" and create a brotli file of it. I used Peazip, and here I run into the problem that the default compression level of "3" did not work (viewing the file in browser now was possible, but it was cut). It worked when using compression level "11".

Workaround 3:
Disable static compression: add this to your project file:

<PropertyGroup>
  <CompressionEnabled>false</CompressionEnabled>
</PropertyGroup>

This will disable compression completely, so it might be bad for performance.

[omer-repo]

Issue with modifying appsettings.json after publish (.NET 10 – Blazor Hosted WebAssembly)

I’m experiencing what seems to be the same issue on .NET 10 with Blazor Hosted WebAssembly (I had a similar situation in .NET 9 as well).

What I observe

• If I publish the project and deploy the entire publish folder to IIS without modifying anything, everything works fine.
• However, if I modify appsettings.json after publish (inside the publish folder) and redeploy without touching the compressed files, I get the following error:

AggregateException_ctor_DefaultMessage (ZeroDepthAtEnd, LineNumber: 28 | BytePositionInLine: 12.)

Important details

• If I modify appsettings.json before publishing, everything works perfectly.
• The .br file generated by Visual Studio during publish matches the JSON content and there are no issues.
• The problem only appears when I change appsettings.json after publish but do not regenerate the corresponding appsettings.json.br.

.NET 9 behavior

In .NET 9, I was able to fix this by manually recompressing appsettings.json using Brotli (via PeaZip) and regenerating the .br file with a high compression level (9 or 11 worked for me). After that, the app ran correctly.

.NET 10 behavior

In .NET 10:

• Manually recompressing does not fix the issue anymore.
• Even when using Brotli, the generated .br file size does not exactly match the one produced during publish (there is always a small difference).
• It seems .NET 10 might be using a different Brotli configuration or additional metadata.

My assumption

My assumption is that during publish, Blazor records or expects a specific relationship between the uncompressed and compressed files (possibly related to static asset fingerprinting, integrity checks, or precomputed metadata).

If appsettings.json is changed without regenerating the compressed asset exactly the same way the SDK does, the runtime fails while parsing.

Summary

• Modifying appsettings.json after publish breaks the app.
• Modifying it before publish works.
• Manually recreating .br worked in .NET 9 but no longer works in .NET 10.

Is there a documented way to safely update configuration files post-publish in Blazor WebAssembly, or is this behavior expected due to static asset processing?

[WolfgangHG]

@omer-repo Did you open https://myserver/MyWebApp/appsettings.json in a browser? Does it return the modified "appSettings.json" or an error or something "strange"?

Also, what happens if you use my "Workaround 1" and disable compression in Firefox?

I actually don't know anything about Blazor, my previous post was the result of a colleague being on vacation ;-)

[omer-repo]

Thanks for taking the time to respond — much appreciated 🙂

Regarding your questions:

When I modify appsettings.json after publish and redeploy, opening
https://myserver/MyWebApp/appsettings.json in the browser does not return the modified content.
In fact, the file does not load correctly at all (it fails), which already indicates something is wrong at the static asset level.

As for "Workaround 1" (disabling compression in Firefox), I haven’t tried it yet.
The reason is that this application is used by many end users, so applying a browser-specific workaround on my side wouldn’t be a viable solution in a production scenario.

From my perspective, this looks more like a publish-time asset integrity issue rather than a browser compression issue — especially since:

• Modifying appsettings.json before publish works perfectly.
• Deploying the untouched publish output works perfectly.
• The issue only appears when the file is modified post-publish.

So it seems related to how Blazor handles static assets and compression during publish.

Still, thank you for pointing me in that direction, you can look at #64923 also.