(process:2503): GLib-WARNING **: GError set over the top of a previous GError or uninitialized memory. This indicates a bug in someone's code. You must ensure an error is NULL before it's set. The overwriting error message was: Error parsing XML data I was attempting to convert from a svgz that indeed had mangled XML inside. But the error indicates some bad coding style in ImageMagick that should probably be checked.
Short intro: I'm a new co-maintainer of ImageMagick and as such walking through all open bugs. This is fixed in 6.4.0.10-1, which should become available in rawhide once it unfreezes from the current F-9 freeze.