On some video files (not all) I get the follow error and the produced file is empty :

$ gifify --colors 255 -o x.gif --resize '800:-1' VID_20170820_124826.mp4
convert-im6.q16: DistributedPixelCache '127.0.0.1' @ error/distribute-cache.c/ConnectPixelCacheServer/244.
convert-im6.q16: cache resources exhausted `/tmp/magick-22271zppvVjFkR5gk' @ error/cache.c/OpenPixelCache/3945.
$ file x.gif 
x.gif: empty
$ file VID_20170820_124826.mp4
VID_20170820_124826.mp4: ISO Media, MPEG v4 system, 3GPP
$ dpkg -l ffmpeg
ii  ffmpeg                 7:3.2.4-1build2  amd64            Tools for transcoding, streaming and playing of m
$ dpkg -l imagemagick
ii  imagemagick            8:6.9.7.4+dfsg-3 amd64            image manipulation programs -- binaries

It seems that issue could be closed, since the workaround described in #396 works great (ie. increasing the available memory in the /etc/ImageMagick-6/policy.xml file).