Major Bug - Linear Color Key Effect Breaks Adobe Media Encoder
Latest version of AE, Media Encoder, Mac Catalina, on an iMac Pro base model.
If an AE project has a layer with the Linear Color Key effect applied, the project will fail to load in Adobe Media Encoder. If the effect is removed from the layer, then it loads in Media Encoder as expected.
The same issue occurs if a layer has Video Copilot Optical Flares plugin applied.
There may be other effects causing this issue, but these are the only two I have definitively identified.

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Alexey Pivovarov commented
Same problem, took me a while to identify what was causing this issue until I found that it was the Linear Color Key.
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Ozzy Eyre commented
Suffering the same issue here. There's a workaround using the render queue that seems to work but it's far from ideal
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Brent King commented
I have been able to reproduce this bug as well. Using latest versions of After Effects and Media Composer.
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Dillon Brannick commented
I'm having this issue with Linear Color Key as well on Windows 10 (on the latest version of AE (17.0.6), was mind numbing trying to figure out what was causing the issue.
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Pedro Aragao commented
Applying a LUD effect also causes the same issue.