Hi, i got the same error converting a pnm created by scanimage to png:
scanimage ... + scanimage --mode=Lineart --progress --resolution 150 --threshold 200 -x 207 -y 295 + convert image_001.pnm image_001.png convert: unable to read image data `image_001.pnm' @ error/pnm.c/ReadPNMImage/617. This error occurs on debian 8.5 since the imageMagick update to 6.8.9.9-5+deb8u4. Regards, Volker Weber