Fix 0XC000021B STATUS_DATA_NOT_ACCEPTED in 5 Minutes

The 30-Second Fix: Restart Your Network

This sounds stupid, but I've fixed this exact error twice this month by just restarting the network stack. Don't overthink it yet.

1. Open Command Prompt as admin (right-click Start, select 'Command Prompt (Admin)' or 'Windows Terminal (Admin)').
2. Type this and hit Enter:
   

   ipconfig /release && ipconfig /renew && ipconfig /flushdns

3. Then restart your machine.

If the error pops up again when you open an app that uses the network (like a browser, Outlook, or SQL Server), move to the next step.

The 5-Minute Fix: Reset the Winsock and TCP/IP Stack

This is where I've seen the 0XC000021B error die 9 times out of 10. The TDI (Transport Driver Interface) client is basically the middleman between your apps and the network hardware. When that gets corrupted—often from a bad VPN install, a crash during a Windows Update, or an antivirus that didn't uninstall cleanly—it throws this error.

Here's the exact sequence I use:

1. Open Command Prompt as admin again.
2. Run this to nuke and reset Winsock:
   

   netsh winsock reset catalog

3. Wait for it to say 'Successfully reset the Winsock Catalog.' Then run:
   

   netsh int ip reset reset.log

4. Close the command prompt and restart your PC.

That reset.log file is just a log of what changed—you can ignore it. After the reboot, test your network. If the error's gone, you're done. If not, it's probably your network driver.

The 15-Minute Fix: Replace the Network Driver

If Winsock reset didn't work, the driver itself is likely hosed. I had a client last month whose Realtek PCIe GbE driver went sideways after a power surge. The error showed up every time they launched QuickBooks. Driver rollback fixed it.

Here's what to do:

1. Press Win + X and select Device Manager.
2. Expand Network adapters.
3. Right-click your active network adapter (usually the one with no red X or yellow triangle) and choose Properties.
4. Go to the Driver tab.
5. Click Roll Back Driver if it's available. If it's grayed out, click Update Driver -> Browse my computer for drivers -> Let me pick from a list of available drivers on my computer. Choose an older version from the list and click Next.
6. Restart your PC.

If rollback isn't an option, uninstall the driver completely (check 'Delete the driver software for this device'), then restart Windows. It'll auto-install a generic driver. You can then download the latest driver from your motherboard or laptop manufacturer's site—skip the Windows Update automatic driver search, it often pulls the same corrupted one.

Pro tip: If you use a VPN like Cisco AnyConnect or OpenVPN, uninstall it before doing the driver fix. Those VPN clients hook into the TDI layer and can corrupt it on uninstall. Reinstall the VPN after the fix.

Still Broken? Run a System File Check

If you've done all three steps and the error's still there, your system files might be the culprit. This is rare but happens after a failed Windows update.

1. Open Command Prompt as admin.
2. Run:
   

   DISM /Online /Cleanup-Image /RestoreHealth

   This takes 5-10 minutes. Let it finish.

3. Then run:
   

   sfc /scannow

4. Reboot.

If that still doesn't kill the error, I'd look at a recent third-party application install. Something that injects into the network stack—like a firewall, parental control software, or even some game overlays (Discord, Steam). Uninstall the most recent non-Windows software you added just before the error started showing up.

Honestly, I've never seen 0XC000021B survive the Winsock reset and driver rollback combo. That's your best bet. Good luck.