This has worked just fine for me several times before, but this time though i keep getting an error message. It gets about 3 fourths of the way though and says:
"Could not complete this operation. An unknown error has occurred."
I have closed and restarted Photoshop with the same results. I have rebooted my laptop with the same results. I have re-rendered the png stack out of After Effects, with the thought that one png might be corrupted. Still the same results.