0x800401E9 — Unknown interface / COM error while opening a document The document handler exposed an interface the caller does not implement.
Why it happens
- A shell extension conflicts with the document handler
- An Office add-in replaced the document handler
- A 32/64-bit add-in mismatch
What to try
- Disable shell extensions and test again
- Disable Office add-ins
- Match the add-in bitness to the host
One thing to keep in mind
Turn off shell extensions you do not use.
If the same code appears after a Windows update, the driver and permission pages here are worth a read before you reinstall anything: drivers, error codes.
Checked 2026-10-05 — R. Okonkwo.
Before changing anything
The code above is what an application reports; it is not a diagnosis of what went wrong. Two checks are worth doing before permissions are touched: whether the file is on a volume the application can actually reach, and whether the file was moved or renamed since it was last opened. Both report as this code more often than a permission problem does.
Pages in this section describe the code as applications report it. Where a fix is listed here, it is one the library has seen work on a real file; where nothing has been confirmed, the page says so instead of guessing at a cause.