Localhost applications failing after installing 24H2/25H2 BUILD for x64-based Systems

Hi @mohamed irfan,

Thanks for updating your situation!

I'm glad to hear that your issue has been resolved. I truly appreciate your time and effort in finding a resolution. I believe that many users may encounter similar issue, and your case could serve as a valuable reference for them.   

Therefore, I would like to make a brief summary of this post if you don't mind so that other forum members could easily find useful information here:

Based on OP (Original Poster) initial description, the issue started after upgrading from Windows 11 23H2 to 24H2/25H2, where IIS sites on localhost returned HTTP Error 500.0 – Internal Server Error. This typically points to configuration parsing or module handling changes introduced in newer builds.

From what OP found, the root cause seems tied to stricter handling of configuration syntax in IIS after the update. Removing special characters from the IIS configuration file resolved the problem.

Again, I really appreciate your time and effort updating your post with a resolution.

If you find this helpful, please kindly consider marking this as a final answer by following this instruction so others can seek help from your post.

Thank you!

Hi @mohamed irfan,

Thanks for your details and screenshot!

From what you described, this is a known Windows issue introduced by recent updates. It affects IIS and other apps using HTTP.sys when handling HTTP/2 on localhost, even if your app didn’t change.

Microsoft has acknowledged this in their KB articles in the "IIS websites might fail to load" section:

• KB5065789
• KB5066835

I would recommend you try these workarounds first:

1. Update first (recommend)

• Go to Settings → Windows Update → Check for updates, then restart. Microsoft’s Known Issue Rollback (KIR) or cumulative fix will apply automatically.
• If you manage devices via Group Policy, deploy the KIR policy from Microsoft’s guidance.

2. Disable HTTP/2 for IIS

Add these registry keys and restart IIS:

• [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\IIS\Parameters]
• "EnableHttp2"=dword:00000000

Then run: iisreset.

For more details: Fix Windows 11 KB5066835 localhost bug

3. Optional: Disable HTTP/2 globally (HTTP.sys)

If IIS-only doesn’t help, use:

• [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters]
• "EnableHttp2Tls"=dword:00000000
• "EnableHttp2Cleartext"=dword:00000000

Then reboot.

For more details: How to disable HTTP/2 on IIS 10

Disclaimer: Some of these links are non-Microsoft website. The pages appear to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classifies as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.

In short: your app didn't break but the OS changed how HTTP/2 works on localhost. Update and restart first; if that doesn’t fix it, temporarily disable HTTP/2 for IIS or globally. Both are safe and reversible.

I hope this helps! If you have any questions, please comment below. I'll be happy to help!