Wand Policy Error: error/constitute.c/ReadImage/412
Troubleshooting ImageMagick on Debian-based systems.
If encountered with a PolicyError, you may be able to fix it by changing the following line in /etc/ImageMagick-6/policy.xml from this:
<policy domain="coder" rights="none" pattern="PDF" />
to this:
<policy domain="coder" rights="read|write" pattern="PDF" />
(More details about policy.xml available here.)
Taken from the repo
This is probably due to an security fix of the underlaying package (see https://bugs.launchpad.net/ubuntu/+source/imagemagick/+bug/1796563)
I fixed it for me by editing the /etc/ImageMagick-6/policy.xml and changed the rights for the pdf line to "read":
<policy domain="coder" rights="read" pattern="PDF" />