Fix RPC_S_INVALID_ENDPOINT_FORMAT (0X000006AA) Error

1. Corrupted RPC Endpoint Mapper Registry Key

This is the one I see most—maybe eight out of ten times. The error code 0X000006AA means Windows is trying to parse an RPC endpoint string and the format is garbage. Usually happens after a bad uninstall of some network software, or a forced shutdown during a Windows update. Had a client last month whose entire print queue died because of this after a power outage.

The endpoint mapper keeps its data in the registry under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rpc. If that key gets corrupted or has a malformed subkey, you'll see this error when any RPC-dependent service tries to start—like the Print Spooler, Remote Registry, or even File Server Resource Manager.

Here's the fix:

1. Open regedit.exe as Administrator.
2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rpc.
3. Look for any subkeys that look wrong—nonsensical names, empty values, or entries with weird characters. A common one is a subkey named something like RpcEndpointMaper (misspelled) or just garbage text.
4. Right-click each suspicious subkey and export it as a backup first. Then delete it.
5. Close regedit and restart the RpcSs service: open Command Prompt as Admin and run net stop RpcSs && net start RpcSs.

If the error goes away, you're golden. If not, the corruption might be deeper—restore from that backup you made and try the next fix.

Pro tip: Don't delete the entire Rpc key. Only remove subkeys you're sure are bad. I've seen people nuke the whole thing and then need to rebuild the RPC stack from scratch. Not fun.

2. Third-Party Software Corrupting the RPC Endpoint Mapper

This one sneaks up on you. Some VPN clients, antivirus suites, or even printer drivers inject their own RPC endpoint definitions. When they uninstall incorrectly, they leave behind half-removed entries that point to DLLs that don't exist anymore. The RPC service then tries to load those endpoints and chokes with 0X000006AA.

The usual suspects: Symantec Endpoint Protection (old versions), Cisco AnyConnect VPN, and HP printer drivers. HP is notorious for this—I've pulled junk from their installations out of the Rpc key more times than I can count.

How to check:

1. Open Event Viewer (eventvwr.msc), go to Windows Logs > System.
2. Look for events with source RpcSs or RPC around the time the error happened.
3. Read the details—often they'll mention a specific DLL or service name that's triggering the bad endpoint.

Once you've identified the culprit software:

• Uninstall it cleanly using Revo Uninstaller (free version works fine) to nuke leftover registry entries.
• Then manually search the registry for any remaining references to that software's DLLs under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rpc and delete them.
• Reboot and test.

One last thing: if it's a printer driver, don't just remove the printer. Use Print Management (printmanagement.msc) to delete the driver itself from the server's driver store. That's where the bad RPC endpoint entries live.

3. Corrupted RPC Service Files or Missing DLLs

Rarest of the three, but it happens. The RPC service itself (RpcSs) relies on rpcrt4.dll, rpcepmap.dll, and a few others. If one of these gets corrupted—usually from a bad Windows update or a disk error—the service can't parse endpoints correctly.

You'll know you're here if the first two fixes did nothing and the error persists even in Safe Mode.

Fix:

1. Open an elevated Command Prompt.
2. Run sfc /scannow. Let it finish—it'll replace any corrupted system files it finds.
3. Then run DISM /Online /Cleanup-Image /RestoreHealth. This fixes the image store that SFC uses.
4. Reboot after both complete.

If SFC finds corrupt files but can't repair them, your image store is likely broken. In that case, run a Windows 10/11 in-place upgrade from the Media Creation Tool—it'll reinstall the OS while keeping your apps and files. That almost always fixes RPC file corruption.

Real-world example: Back in 2022, a botched KB5006670 update on Windows Server 2019 broke rpcrt4.dll for a few people. The error code was exactly this one. The in-place upgrade was the only thing that worked.

Quick-Reference Summary Table

Start with the registry fix. Then check for leftover software. Only go nuclear on system files if nothing else works. This error is annoying but not fatal—once you clean up the endpoint map, everything comes back.