Okay, I've started looking at the builder a little more:

jackhill@alperton ~$ cat /gnu/store/larqpc2wjhnc6jmj4885k8lynd19fl4m-grub-image.png-builder 
(if (string-suffix? ".svg" "/gnu/store/83qplqmavzphd30hm1maxwlh166ylpwr-guix-artwork-2f2fe74-checkout/grub/GuixSD-fully-black-4-3.svg") (begin (use-modules (gnu build svg)) (svg->png "/gnu/store/83qplqmavzphd30hm1maxwlh166ylpwr-guix-artwork-2f2fe74-checkout/grub/GuixSD-fully-black-4-3.svg" ((@ (guile) getenv) "out") #:width 1024 #:height 768)) (copy-file "/gnu/store/83qplqmavzphd30hm1maxwlh166ylpwr-guix-artwork-2f2fe74-checkout/grub/GuixSD-fully-black-4-3.svg" ((@ (guile) getenv) "out")))

The problem appears to be in the svg->png procedure or at least in the 
svg.scm file. On the "bad" system:

jackhill@kalessin ~$ guix environment --ad-hoc guile guile-rsvg guile-readline
jackhill@kalessin ~ [env]$ guile
GNU Guile 3.0.5
Copyright (C) 1995-2021 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> ,use (gnu build svg)
;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
;;;       or pass the --no-auto-compile argument to disable.
;;; compiling /run/current-system/profile/share/guile/site/3.0/gnu/build/svg.scm
;;; WARNING: compilation of /run/current-system/profile/share/guile/site/3.0/gnu/build/svg.scm failed:
;;; failed to create path for auto-compiled file "/run/current-system/profile/share/guile/site/3.0/gnu/build/svg.scm"
scheme@(guile-user)> (svg->png "/gnu/store/83qplqmavzphd30hm1maxwlh166ylpwr-guix-artwork-2f2fe74-checkout/grub/GuixSD-fully-black-4-3.svg" "/tmp/test.png")
ice-9/boot-9.scm:1669:16: In procedure raise-exception:
Wrong type (expecting finalized smob): #<cairo-context 7fb032a3e6b0>

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.

On the good system

ckhill@alperton ~$ guix environment --ad-hoc guile guile-rsvg guile-readline
jackhill@alperton ~ [env]$ guile
GNU Guile 3.0.5
Copyright (C) 1995-2021 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> ,use (gnu build svg)
;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
;;;       or pass the --no-auto-compile argument to disable.
;;; compiling /run/current-system/profile/share/guile/site/3.0/gnu/build/svg.scm
;;; compiled /home/jackhill/.cache/guile/ccache/3.0-LE-8-4.4/gnu/store/0j6w61vjjvp4zqzrqvyhqm6254ppzh8y-guix-1.2.0-16.c8887a5/share/guile/site/3.0/gnu/build/svg.scm.go
scheme@(guile-user)> (svg->png "/gnu/store/83qplqmavzphd30hm1maxwlh166ylpwr-guix-artwork-2f2fe74-checkout/grub/GuixSD-fully-black-4-3.svg" "/tmp/test.png")

and a png file is produced. Particularly relivant seems the 
auto-compilation failure.

To be continued…

Best,
Jack