Thanks Ben. I tried with google but obviously not with right keywords from error message.
I suppose that buyer didn't provided me exact error message, and that's why "After Effects error: A file system I/O error has occurred. (512) (25 :: 60)" (with quotes) on google shows only this thread.
My logic is always: if I have any problem with any software, someone once had the same problem before me, and google must know the answer :)