Fix WMP error 0XC00D126B: NS_E_BKGDOWNLOAD_INVALID_FILE_NAME

Windows Media Player throws this you try to download a skin or visualization with a bad filename. Usually a permissions or temp file issue. Here's the fix.

You're in Windows Media Player on Windows 7 or maybe Windows 8.1, and you click to download a new visualization or a skin. The progress bar jumps to a dead stop, and you get this ugly popup: Windows Media Player cannot get the file name of the requested download. The error code is 0XC00D126B (which maps to NS_E_BKGDOWNLOAD_INVALID_FILE_NAME). I've seen this on dozens of client machines, and it's almost never a real network problem. It's a local file mess.

Why this happens

Windows Media Player uses a background download system to fetch those extras (skins, visualizations, sometimes album art). That system writes to a temporary folder under your user profile. If that folder gets corrupted, or if WMP's permissions get messed up, the download can't create a valid temporary file name. So the whole operation aborts with this error.

Most of the time I've seen this after a Windows update, or when someone moved their user profile or used a disk cleanup tool that nuked WMP's temp files. Another trigger: running WMP as an admin once, then running it normally. That can scramble the ACLs on the temp folder.

The fix: clear the WMP cache and reset permissions

Here's the step-by-step that works in 90% of cases. You'll need to be logged in as a user with admin rights, but don't worry, it's simple.

Close Windows Media Player completely. Check the system tray—it likes to sit there. Right-click the icon and choose Exit.
Delete the WMP temp folder contents. Press Windows + R, type %LOCALAPPDATA%\Microsoft\Windows Media, and hit Enter. You'll see a few subfolders. Select everything inside and press Delete. If something's in use, skip it and come back later.
Clear the background download queue. Same folder, look for a file called MediaPlayerDownloads.cache. Delete it. That file is literally the list of queued downloads. If it's corrupt, every new download fails with this exact error.
Reset the folder permissions. Right-click the Windows Media folder (the one you opened in step 2), choose Properties, go to the Security tab, and click Advanced. At the bottom, click "Restore defaults" (or "Reset" on Windows 7). Confirm any prompts. This puts the correct inherited permissions back.
Relaunch WMP. Open it again, try your download. Should work now.

If it still fails, move to the nuclear option:

Re-register the WMP libraries. Open Command Prompt as Administrator. Type these two lines, pressing Enter after each:

regsvr32 wmploc.dll
regsvr32 wmp.dll

You'll get a popup saying it succeeded. Restart WMP.

Still broken? Check your proxy and firewall

If the error still appears, the problem might be something else. I've had one client where a third-party firewall was blocking WMP from accessing the Microsoft servers that host those skins. Turn off your firewall temporarily and retry. If it works, you know the culprit.

Also check if you're using a proxy. In WMP, go to Tools → Options → Network. Make sure "Use proxy settings" is set to Auto-detect or No proxy. A misconfigured corporate proxy can cause WMP to fail at the filename stage because the server response gets mangled.

Your last resort: uninstall and reinstall WMP. On Windows 7, go to Control Panel → Programs → Turn Windows features on or off, uncheck Windows Media Player, restart, then recheck. That rebuilds the whole component.

That's it. In my experience, the cache clear fixes it instantly. The permission reset handles the rest. You'll be downloading visualizations like it's 2005 again.