I found that that every time I import a Premiere sequence, AE will import the sequence and all respective footage, markers, etc correctly, but if the sequence includes any Crop effects from Premiere, AE will report an unknown error.

If I open that imported sequence in AE, I see all the respective layers from Premiere, but no image forms in the viewport. It's as if all layers were at 0% visibility, and this cannot be fixed. The only solution is to go back to Premiere and delete the crop effect before importing to AE.

I recently had a project where I had 30 video tracks in Premiere, all of them with different crop values across the sequence, some animated. I had to redo all this work in AE.