You need to be much more specific - render settings, file format, save locations, version info for both programs, system info And so on. Just saying there's a problem is no use to anyone.
I am also getting the same problem. It happens when I import an ae project file into another ae project file.
I am running
os x el capitan
Mac Pro late 2013
Adobe AE CC 2015.3
Trashing my preferences hasn't helped.
Restarting my computer hasn't helped... let's see if the ae update will...
Updating to ae cc 2017 (220.127.116.11) didn't work.
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I found thenproblem and solution, in our scenario at least.
The image sequence out of C4D was naming the files like this "_ao0001". i simply renamed all files using Automator to change the name to "ao_0001" and it solved it; you can easily batch rename everything all at once in a folder.
I guess AE doesnt like characters directly in front of numbers. Hope this works for everyone else!
I'm using the newest version of After Effects (15.0.1) on MacOS 10.13.2 and I'm also getting this error every time I try to import an AEP into my project file.