If the error occurs upon launching After Effects, a script in your startup folder is likely the culprit.
in custom pseudo-effects or Master Properties, as these can cause structural conflicts. Advanced Troubleshooting Isolate the Culprit: If the error occurs upon launching After Effects,
: On both macOS and Windows, ensure After Effects has "Read & Write" access to its own folders. Restricted permissions can prevent scripts from creating the properties they need to function. Common Triggers to Watch For "unknown property of type 'Mask Feather'")
If the error message includes a specific property name (e.g., "unknown property of type 'Mask Feather'"), use that clue to inspect your project for that specific property. Deleting and re-creating that specific property (e.g., removing a mask and drawing a new one) often resolves the issue immediately. If the error occurs upon launching After Effects,