I've run into this problem with two different jobs at two different companies (one on Mac and one on Windows) with AE CC2014. I simply cannot save out a still (Comp>Save Frame As >File...) as a PNG. I have output templates for PNG 8-bit RGB, RGBA and 16-bit RGB and RGBA. Tried all of them. Always get this error message about PNGIO.
Has really put a dent into my usual workflow. Anyone else having this problem?
Known issue. Some fix is floating in the stars.