All Animation Blueprints Fail Cook

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Ever since I updated the engine from 4.18 to 4.27.2, all 17 of my project’s animation blueprints fail cooking with the following error & warnings:

PackagingResults: Error: UStructProperty::Serialize Loading: Property ‘StructProperty /Game/Game/Player/AnimBP_Hand.AnimBP_Hand_C:AnimGraphNode_Root_5D86027148C1978FFE7878B5829499C1’. Unknown structure.

PackagingResults: Warning: Property AnimGraphNode_Root_5D86027148C1978FFE7878B5829499C1 of AnimGraphNode_Root_5D86027148C1978FFE7878B5829499C1 has a struct type mismatch (tag AnimNode_Root != prop FallbackStruct) in package: …/…/…/MyProj/Content/Game/Player/AnimBP_Hand.uasset. If that struct got renamed, add an entry to ActiveStructRedirects.

I’ve seen other posts regarding this topic, but my issue here is different in these ways:

  1. As a test, I get the above error even when I create a fresh anim BP that is completely blank (the default when you first create it). There are no nodes in the Anim Graph or Event Graph, no variables, no functions, etc. It’s completely blank.

  2. My character BP that uses the above blank anim BP is also blank with the exception that it specifies the skeletal mesh and the blank anim BP. There’s absolutely nothing else in this char BP.

  3. If I create the above blank anim BP & char BP in a stock 4.27.2 project instead of my project, it cooks fine. If I Migrate those over to my project, I get the above error.

  4. The game runs fine in PIE, despite the error.

I have not modified the engine. I’ve tried everything I can think of including a clean / rebuild of the engine and game.