Well, I added <policy domain=”coder” rights=”read | write” pattern=”PDF” /> just before </policymap> in /etc/ImageMagick-7/policy.xml and that makes it work again, but not sure about the security implications of that.
Note: the solution in this and other answers involves disabling safety measures that are there to fix arbitrary code execution vulnerabilities. See for instance this ghostscript-related and this ubuntu-related announcement. Only go forward with these solutions if the input to convert comes from a trusted source. I use ImageMagick in php (v.7.1) to slice PDF … Read more