Graechan, I'm not sure what problem you had but there are some issues with the APNG plug-in.
Note from the plugin DeveloperQuote:
First frame must be full size (and second frame must be full size too, if first frame is hidden).
Failure to follow this convention will result in a save error..
libpng error: width and/or height in the first frame's fcTL don't match the ones in IHDRI received this error when the top layer in my layer dialog was not the same size as the overall image.
Once I resized the top layer to the image size, I was able to save as an animated PNG.
As far as I know, that plug-in hasn't been updated since 2010.