ERROR_FLT_NO_WAITER_FOR_REPLY (0x801F0020): Fix It Fast

What ERROR_FLT_NO_WAITER_FOR_REPLY (0x801F0020) Actually Means

The full name is ERROR_FLT_NO_WAITER_FOR_REPLY, and the numeric value is 0x801F0020. It comes from the Windows Filter Manager (fltmgr.sys), which manages minifilter drivers — the little kernel modules that antivirus tools, backup software, and encryption programs hook into. When a minifilter finishes a request, it sends a reply back to whoever asked. If the original requester has already given up, timed out, or crashed, there's no waiter for the reply. The Filter Manager returns this code.

You'll usually see it in a few places: Windows Event Viewer under the System log, in vendor logs from backup tools like Veeam, Acronis, or Macrium Reflect, or as a popup from antivirus software. A very common real-world trigger is a backup job that runs while an antivirus scan is also running. Both use minifilters, one cancels a request, and the other still tries to reply. Boom — 0x801F0020.

Don't panic. This is almost never hardware. It's a coordination problem between drivers. Work through the fixes in order below and stop as soon as your issue clears up.

Fix 1: Reboot and Retry (30 seconds)

Yes, really. A stale minifilter context can linger in memory after a crashed backup or a killed scan process. A clean reboot clears the Filter Manager's waiter queue and reloads every minifilter from scratch.

1. Save your work and close everything.
2. Click Start, then Power, then Restart. Don't use Shut Down — Windows 10 and 11 do a hybrid shutdown by default that keeps the kernel session alive, and that can preserve the broken state.
3. Wait for the desktop to come back. Give it a full minute so all the minifilters finish loading.
4. Rerun whatever operation triggered the error — the backup job, the file copy, the app that failed.

Expected outcome: the operation completes without the error. If it comes back immediately, move on.

Fix 2: Identify and Unload the Offending Filter (5 minutes)

Windows ships with a built-in tool called fltmc that lists every minifilter loaded right now. Open an elevated Command Prompt (right-click Start, choose Terminal (Admin) or Command Prompt (Admin)) and run:

fltmc filters

You'll get a table with columns for Filter Name, Num Instances, Frame, and Altitude. The ones you care about are third-party filters — they usually have vendor-ish names like avc3 (Avast), wdcfilter (Webroot), SymEFA (Symantec), mfehidk (McAfee), or something like vhdxflt for Veeam. High-altitude numbers (above 300000) sit close to the filesystem and tend to cause this error more often.

To unload a filter without rebooting, run:

fltmc unload <filter_name>

For example:

fltmc unload avc3

If the unload succeeds, you'll just see a new prompt. Try your operation again. If it errors with "The specified filter cannot be unloaded" or "Access is denied," that filter is either in use by a running process or marked as non-unloadable.

Expected outcome: once the offending filter unloads, the error stops. Which filter to try first? Start with whatever antivirus or backup product you installed most recently. The newest minifilter is the most common culprit.

Don't unload FileInfo, WdFilter, FsFilter, or anything starting with bindflt unless you know exactly what you're doing. Those are Microsoft's own and Defender relies on them.

Fix 3: Update or Reinstall the Problem Driver (15+ minutes)

If unloading works but the filter reloads on every boot and the error comes back, you're dealing with a version conflict. The real fix is updating the driver, or if that fails, replacing it.

Update the driver

1. Open Device Manager (devmgmt.msc).
2. Click View, then Show hidden devices.
3. Expand Non-Plug and Play Drivers. You should see the vendor's driver listed there.
4. Right-click it, choose Update driver, then Search automatically.

For antivirus and backup tools, updating through the app itself is more reliable. Open the tool, check for updates, install, and reboot.

If that doesn't help, reinstall the filter

Uninstall the product using its own uninstaller — not Add/Remove Programs unless the vendor tells you to. Most security suites include a dedicated removal tool. Run it, reboot, confirm the filter is gone with fltmc filters, then reinstall the latest version from the vendor's site.

Expected outcome: the new driver version handles reply cancellation correctly, and the error stops appearing in Event Viewer.

Fix 4: Check Event Viewer for the Real Trigger

If none of the above fixed it, you need to see what's actually happening. Open Event Viewer (eventvwr.msc), go to Windows Logs, System, and filter for source fltmgr or FilterManager. Look for events right before the 0x801F0020 entries. You're hunting for a file path, process name, or filter name.

A pattern I've seen many times: a scheduled backup (Veeam, Windows Server Backup, Macrium) fires at 2 AM, an antivirus scan also starts at 2 AM, and both touch the same files. One driver cancels, the other replies, and the Filter Manager logs the error. Fix: stagger the schedules. Move the backup to 2 AM and the scan to 4 AM. That alone clears this error for a lot of people.

Fix 5: Run System File Checker and DISM (15 minutes)

Rarely, the Filter Manager itself gets corrupted. Run these two commands in an elevated prompt, in this order:

sfc /scannow
dism /online /cleanup-image /restorehealth

SFC takes about 10 minutes. DISM takes another 5 to 15 depending on your disk speed. If either one reports repaired files, reboot and test again. Expected outcome: any corrupted fltmgr.sys or related binaries get restored from the component store.

When to Stop and Call the Vendor

If you've cycled through a reboot, unloaded every third-party filter, updated drivers, and still see 0x801F0020 every few hours, the bug is in the minifilter's code. That's the vendor's problem, not yours. Collect the System event log export, the exact fltmc filters output, and the timestamps of the errors, then open a ticket. Saying "my minifilter at altitude 320000 is replying after its waiter has been cancelled" gets you to a real engineer fast.

Most readers fix this in Fix 1 or Fix 2. If you made it past Fix 3, the odds are you're chasing a driver bug that a vendor update will eventually patch. Don't tear your system apart looking for a hardware fault — this error is driver coordination, full stop.