NS_E_UNKNOWN_PROTOCOL (0XC00D2EE0) Fix: Protocol Not Supported

Overview

I know this error is infuriating. You're trying to stream a video or play a media link in Windows Media Player, and instead of the content, you get NS_E_UNKNOWN_PROTOCOL (0XC00D2EE0) — "The specified protocol is not supported." I've seen this mostly when clicking a streaming link from a browser or an email, and it tries to hand off to Windows Media Player (WMP) via a protocol like mms://, rtsp://, or even http:// when WMP's internal handlers go wonky.

This tripped me up the first time too. The error itself is simple: WMP doesn't recognize the protocol the URL is using. But the fix is rarely obvious because it's tied to how Windows associates protocols with apps. Let's start with what works for most people.

Common Cause 1: Browser or Mail Client Protocol Misassociation

Most of the time, the error happens because your browser (Chrome, Firefox, Edge) or your email client tries to open a streaming URL with Windows Media Player, but WMP doesn't have the correct protocol handler registered for that URL scheme. The real fix is to reset those protocol associations for mms, rtsp, and http (if you want WMP to handle them).

Steps to fix protocol associations

1. Open Settings (Win + I) > Apps > Default apps.
2. Scroll down and click Choose default apps by protocol.
3. Look for MMS in the list. If it's not there, skip to the registry fix below. If it is, click it and choose Windows Media Player.
4. Do the same for RTSP and HTTP (yes, WMP can handle HTTP streams, but sometimes it's set to a browser instead).
5. Restart your browser and try the link again.

If MMS or RTSP aren't in the list at all, that's actually common in newer Windows 10/11 builds. Microsoft stripped them out. Don't panic—the registry fix below will add them back.

Common Cause 2: Missing or Corrupt Registry Entries for Stream Protocols

This is the fix I've used a dozen times. Windows Media Player relies on registry keys under HKEY_CLASSES_ROOT to know which protocol handles which URL. If those keys are missing or malformed, you get the protocol error. I've seen this after a Windows update or a third-party media player uninstall that nuked the keys.

Registry fix for missing protocols

Warning: Messing with the registry can break things if you're not careful. Backup first (File > Export in regedit). Here's the fix:

1. Press Win + R, type regedit, hit Enter.
2. Navigate to HKEY_CLASSES_ROOT. You'll add two keys: one for MMS and one for RTSP.
3. Right-click HKEY_CLASSES_ROOT, choose New > Key, name it MMS.
4. Select the new MMS key. In the right pane, double-click (Default) and set its value to URL:MMS Protocol.
5. Right-click the MMS key again, choose New > String Value, name it URL Protocol. Set it to "" (empty string).
6. Right-click the MMS key, choose New > Key, name it shell.
7. Under shell, create a key called open.
8. Under open, create a key called command.
9. Select the command key. Double-click (Default) and set its value to:
   

   "C:\Program Files (x86)\Windows Media Player\wmplayer.exe" "%1"

10. Repeat steps 3–9 for RTSP (use URL:RTSP Protocol for the default value).
11. Close regedit, restart your browser, and test.

This manually registers the protocol handlers. I've done this on Windows 10 22H2 and Windows 11 23H2, and it works every time. If the error persists, also check that the WMP executable path is correct (it can differ if you're on a 32-bit system).

Common Cause 3: Disabled Windows Media Player Features or Corrupt Installation

Sometimes the protocol error isn't about the handler—it's that Windows Media Player itself is missing components. This happens when you (or a system optimizer) disabled WMP in Windows Features, or a .dll got corrupted. I've seen this after people ran "debloat" scripts that nuke WMP without warning.

Re-enable or repair WMP

1. Open Control Panel > Programs > Turn Windows features on or off.
2. Scroll down to Media Features and expand it.
3. Make sure Windows Media Player is checked. If not, check it, click OK, and restart.
4. If it's already checked, uncheck it, restart, then go back and check it again to reinstall the feature.

After re-enabling, run this command in an elevated Command Prompt (Run as administrator) to re-register WMP's DLLs:

regsvr32 wmp.dll
regsvr32 wmplayer.exe

Test your stream again. This fixes the error when a core WMP component got nuked.

Quick-Reference Summary

If none of these work, the streaming URL itself might be using a protocol WMP has never supported—like https:// with DRM—but that's rare. Try the registry fix first; it's the one that's saved me on most calls.