The most frequent cause is trying to open a project file in an older version of After Effects than the one it was saved in. While After Effects is generally backwards compatible, it is not always forwards compatible. The Solution Ensure your Creative Cloud app is fully updated.
Opening a project created in a newer version of After Effects. Missing plugins that were used in the original project. Corrupt preference files or cache data. Expression errors that reference non-existent effects. Step 1: Check for Version Compatibility
Hold down Ctrl+Alt+Shift (Windows) or Command+Option+Shift (Mac) while launching the program. The most frequent cause is trying to open
The "Asked to Create Unknown Property" error is a common headache for Adobe After Effects users. It usually pops up when the software encounters data it doesn't recognize within a project file. This often happens when moving projects between different versions of After Effects or when using third party plugins. If you are stuck with this error, here is how to resolve it. Understand the Cause
If the project file itself is slightly corrupted, you can often bypass the error by importing it into a fresh workspace rather than opening it directly. Open After Effects and create a brand new project. Go to File > Import > File. Select the problematic .aep file. Opening a project created in a newer version
If the project uses a plugin you don't have installed, After Effects might struggle to define the properties associated with that plugin. How to Fix
Click "Yes" when asked if you want to delete the preferences file. Step 5: The "New Project" Import Trick Expression errors that reference non-existent effects
⚠️ Always keep "Auto-Save" turned on in your Preferences to ensure you have earlier versions of your work to fall back on if a file becomes permanently corrupted.
After Effects will often "repair" the property links during the import process.