convert-im6.q16: DistributedPixelCache '127.0.0.1' @ error/distribute-cache.c/ConnectPixelCacheServer/244.
convert-im6.q16: cache resources exhausted `/tmp/magick-27777Al6FGY7dhyKt10' @ error/cache.c/OpenPixelCache/3984.
convert-im6.q16: DistributedPixelCache '127.0.0.1' @ error/distribute-cache.c/ConnectPixelCacheServer/244.
convert-im6.q16: cache resources exhausted `/tmp/magick-277772Y_-pJnMdT2r1' @ error/cache.c/OpenPixelCache/3984.
convert-im6.q16: DistributedPixelCache '127.0.0.1' @ error/distribute-cache.c/ConnectPixelCacheServer/244.
convert-im6.q16: cache resources exhausted `/tmp/magick-27777m6KQRhWOtkWp1' @ error/cache.c/OpenPixelCache/3984.
convert-im6.q16: DistributedPixelCache '127.0.0.1' @ error/distribute-cache.c/ConnectPixelCacheServer/244.
convert-im6.q16: cache resources exhausted `/tmp/magick-27777ERzzOPUYQMc11' @ error/cache.c/OpenPixelCache/3984.
vi /etc/ImageMagick-6/policy.xml
Set values like this (increase as necessary):
<policy domain="resource" name="area" value="6GiB"/>
<policy domain="resource" name="disk" value="6GiB"/>
<policy domain="resource" name="memory" value="4GiB"/>
<policy domain="resource" name="map" value="4GiB"/>
Be sure not to set levels that are bigger than your disk size and available RAM.