Fix STATUS_FLT_CONTEXT_ALLOCATION_NOT_FOUND 0xC01C0016

You're running a backup job or antivirus scan on Windows 10 or Server 2019, and suddenly you get hit with STATUS_FLT_CONTEXT_ALLOCATION_NOT_FOUND (0xC01C0016). The exact message says "No registered context allocation definition was found for the given request." This isn't a random crash — it's the filter manager telling you a minifilter driver tried to allocate a context that was never registered. I've seen this most often with third-party backup tools (Veeam, Acronis) or aggressive antivirus software (McAfee, Symantec) that hook into the file system.

What Actually Causes This

Every minifilter driver that wants to attach contexts to stream handles, file objects, or volumes must first register those context types with the filter manager via FltRegisterFilter. If the driver skips that step, or if the registration gets corrupted, any FltAllocateContext call will fail with this exact error. The driver's basically trying to use a parking spot that doesn't exist on the lot.

The typical triggers are:

An incomplete or failed driver update that left the context registration registry entry missing
A third-party filter driver loading in the wrong order — often happens after a Windows update
Registry corruption under HKLM\SYSTEM\CurrentControlSet\Services\<DriverName>\Instances
A legacy filter driver not designed for the current Windows version (common after an in-place upgrade)

The Fix — Step by Step

Skip the typical "run SFC" nonsense. That rarely touches minifilter context registrations. Here's what actually works.

Step 1: Identify the Problem Driver

Open Event Viewer (eventvwr.msc) and go to Windows Logs > System. Look for an event from source FltMgr with Event ID 37 or 41. The description will include the driver name (e.g., Symantec, Veeam). Alternatively, run this from an admin prompt:

fltmc instances

This lists all registered minifilters. Look for instances with Altitude values but no Frame assignment — those are your suspects. Also check for any filter showing a Not Started or Stopped state.

Step 2: Check the Filter's Context Registration

For the suspect driver, open Regedit and navigate to:

HKLM\SYSTEM\CurrentControlSet\Services\<DriverName>\Instances
<DriverName> Instance

Look for a ContextSize value under the Altitude key. If it's missing or set to 0, that's your smoking gun. Some third-party installers just don't write this value correctly. You can fix it by adding a REG_DWORD entry named ContextSize with a value of 1 (for a single context) or whatever the driver's documentation specifies. If you don't know the correct size, set it to 1 first — most drivers can handle that.

Step 3: Re-register the Filter Driver

This forces the filter manager to re-read the context registration. Open an admin command prompt and run:

fltmc unload <DriverName>
fltmc load <DriverName>

If fltmc unload fails with access denied, you'll need to uninstall and reinstall the driver completely. For antivirus software, this means disabling real-time protection, rebooting into Safe Mode, and running the vendor's cleanup tool before reinstalling.

Step 4: Verify the Fix

Reboot the machine. Then reproduce the operation that triggered the error — run your backup job or full antivirus scan. If the error is gone, you're done. If not, move to the next section.

When the Standard Fix Doesn't Work

Sometimes the driver registration is fine but the filter manager itself has a corrupted internal state. This is rare but happens after a botched Windows update. Here's what to try:

Run fltmc filters and see if the minifilter shows Frame with a value of 0. If it does, the driver loaded but couldn't attach to a frame. You might need to change its Altitude in the registry to a less common value (e.g., 180000 for antivirus) to avoid conflicts.
Check for orphaned filter instances with fltmc detach <DriverName> -volume C: (replace C: with the volume). Then reattach with fltmc attach <DriverName> <Altitude> C:. This manually forces the context registration.
Use Process Monitor to trace FltAllocateContext calls. Filter by FltMgr.exe and look for BUFFER OVERFLOW or INVALID_PARAMETER results — they'll point to the exact line in the driver's code. That's advanced but it'll save your bacon when nothing else works.

If you're still stuck after all that, the filter driver is almost certainly incompatible with your current OS build. Check the vendor's support site for a newer version. I've had to roll back Windows updates more than once to get legacy backup software working again.