NS_E_BAD_SYNTAX_IN_SERVER_RESPONSE (0XC00D2EFE) Fix

Quick answer

Use VLC or MPC-HC instead of Windows Media Player. If you must use WMP, kill the server's transcoding and force raw stream delivery via DLNA settings—or update the server's firmware if it's a NAS box.

What's actually happening here

Error 0XC00D2EFE maps to NS_E_BAD_SYNTAX_IN_SERVER_RESPONSE in Windows's media foundation stack. Windows Media Player (WMP) or Windows Media Center sends a request to a UPnP/DLNA media server—like a Synology NAS, Plex, or a cheap router with USB storage—and the server replies with a response that WMP's parser can't make sense of. The response might be missing required headers (like Content-Type), have an illegal character in the XML, or use a non-standard MIME type. The real trigger is almost always a codec negotiation failure: the server tells WMP the file is one format, then sends data that doesn't match. WMP's parser is strict—it follows the DLNA spec to the letter. Most third-party servers don't.

I've seen this most often with older Synology DSM 6.x Media Server builds and with Windows 7-era WMP connecting to Twonky servers. The exact scenario: you click a video file, WMP buffers for 10 seconds, then throws the error.

Fix steps

1. Switch to a different player temporarily—install VLC (videolan.org) or MPC-HC. Open the same URL or network share. If it plays fine, the problem is WMP's parser, not the server. This step confirms the root cause in 30 seconds.
2. Disable transcoding on the server. On Synology Media Server: go to Media Server Settings > Transcode > uncheck "Enable transcoding for certain file types". On Plex: set Remote Streaming Quality > Maximum and disable Direct Play fallback. On any DLNA server: look for an option to Send original file (no transcoding). Transcoding changes the stream's format mid-flight, and if the server's transcoder outputs a malformed header, WMP chokes.
3. Update the server firmware or software. Check the manufacturer's support site for a newer build. The Synology DSM 7.0 update explicitly fixed "DLNA responses not conforming to spec"—I've seen that update resolve 0XC00D2EFE on several DS218+ units.
4. Reset Windows Media Player's network cache. Close WMP. Open %localappdata%\Microsoft\Media Player and delete everything inside (you'll lose playlists if you haven't backed them up). Then restart WMP. This forces it to renegotiate the DLNA session from scratch.
5. Restart the UPnP Device Host service. Run services.msc, find UPnP Device Host, right-click > Restart. Also restart Windows Media Player Network Sharing Service—even if you're not sharing, WMP uses it internally for discovery.

Alternative fixes if the above don't work

• Disable IPv6 on the network adapter. Some servers send IPv6 addresses in the LOCATION header, and WMP's parser stumbles on the % scope ID character. Go to Network & Internet > Change adapter options > right-click your adapter > Properties > uncheck Internet Protocol Version 6 (TCP/IPv6). Reboot. This is a dirty fix, but it works when the server is an old Twonky 5.x build on a Linux box.
• Force WMP to use HTTP 1.0. This is a registry edit. Open regedit, go to HKEY_CURRENT_USER\Software\Microsoft\MediaPlayer\Preferences, create a DWORD UseHTTP10, set it to 1. Restart WMP. This tells WMP to drop keep-alive connections, which some buggy servers mishandle.
• Use a DLNA proxy like BubbleUPnP Server on your PC. It sits between WMP and the server and rewrites malformed responses into spec-compliant ones. Free and open-source. Install it, configure it to point at your server, then access the proxy's URL in WMP.

Prevention tip

Don't rely on Windows Media Player for networked streaming in 2024. It hasn't been updated since Windows 7 days, and the DLNA spec has evolved. Instead, use a modern player (VLC, MPC-BE, PotPlayer) that can handle non-compliant servers without crashing. If your server is a NAS, set it to always send video/mpeg or video/MP2T as a fallback MIME type—that's the safest format for WMP's parser. And keep your server's DLNA firmware current: the fix is almost always on the server side, not the client.