Find the expression, kill the weird code, or re-build the master property. You'll be back to keyframing in less than five minutes.
You are using a plugin (like Red Giant Trapcode, Video Copilot Element 3D, or a script like Duik Bassel). The plugin creates "Pseudo Effects" (custom user interfaces). If the plugin version mismatches the project version, or the plugin is missing, AE cannot create the UI property. Find the expression, kill the weird code, or
When this box pops up, your RAM preview stops, your render fails, and your heart sinks. What on earth is a "FIX" property? Find the expression
Now that we've covered the causes of the error, let's dive into the solutions: kill the weird code
Use in a copy of your project: