Re-download from a different source. Many websites stored incomplete SWFs during the Flash era due to partial caching. Use a tool like "SWF File Verifier" to check header integrity.
Here are the three best ways to play today:
Adobe released a standalone "Flash Player Projector" (also called the debug player). It is safe because it does not have browser integration.
With the death of Flash in browsers, you cannot simply "play online" anymore. Most major browsers (Chrome, Firefox, Edge, Safari) block Flash content by default or have removed support entirely. Thus, has become the only reliable method to experience thousands of classic titles.