Final cut Pro and Action Essentials 2

It's the codec (Animation) of those files that's not meant for real-time playback. Especially since it has alpha channels (so you can composite explosions 'n stuff over your footage) which simply require too much processing/hard drive speed for most systems to playback without stuttering.

For stutter-free playback, you'll need to render the footage in FCP. If that burns too much time (and that's usually the case), drop all your RT settings to Low and turn off the Drop Frame warning.

can you nest the effect and export it out? or try to composite the effect in motion maybe?

What I do with animation codec stock shots-- unless they're being composited with alpha channels-- is drop them into compressor and render them out to what I'm otherwise working with. If you're doing compositing, you would typically save that stuff for last in the workflow and just budget yourself plenty of time for renders etc. It's all billable hours right?

I was mistaken, the VideoCopilot stuff is actually PNG codec, not Animation. (PNG codec files are smaller files, but FCP cannot play them at real-time +at all+; in that way they are worse than Animation codec files.)

So what what happens when you try to render in your current sequence? Still a General Error? (You mentioned exporting, but not rendering, and though they are related sometimes one works when the other doesn't.)

Also, have you attempted to convert the AE2 files -- using Compressor or QT Pro -- into either Animation or ProRes 4444 files? Sometimes that helps.

as a few of us said, try compositing in Motion or After Effects.. Then render that out...