Tools like ClrMamePro can scan your library and identify exactly which files are missing or named incorrectly.

This file is almost always associated with third-party software , specifically older or poorly coded applications related to:

Use a two-step approach:

: Using a ROM set intended for an older version of MAME with a newer emulator executable often leads to "NOT FOUND" errors for newly required support files. How to Fix the Error

The .key extension typically denotes a license key or an encrypted authentication file. The prefix xmvsf likely stands for a proprietary codec pack or a “Video Streaming Framework.” When a legitimate program needs to verify your license, it looks for this file. If it is missing, moved, or corrupted, the program throws the "xmvsf.key not found" error.