Affinity Designer exports SVG files that do not load correctly in VCV Rack. The main problem is that the are placed at the end of the file when the renderer of Rack requires them in the beginning.
Your require OCaml with at the library xml-light and ocamlbuild. You can install them using opam.
$ ocamlbuild -use-ocamlfind fixsvg.byte
To fix a single file:
$ fixsvg.byte file.svg
To fix all files in a directory
$ fixsvg.byte -dir /path/to/files