ImageMagick ghostscript 'delegate' security policy blocking conversion
<policy domain="delegate" rights="none" pattern="gs" />
needs to be commented out. This is now noted on the imagemagick page of the arch wiki. That page currently cites these bug reports: FS#59778, FS#62171
For some reason, this didn't immediately work for me. Somehow I got it to work today by moving policy.xml to a backup (deleting), running the convert command, and moving back the original with the line above commented out.