Green stroke problem

[Ofnuts]

We are ready to help, but show us the file you load in Gimp so that we can try ourselves. What I see otherwise is that both old and new have terribly pixellated edges and I doubt that Blender is the culprit. If you use screenshots, then the old PC may not have the same screen definition as the new and this may be the start of an explanation.

Note that the method I expounded on reddit is also available as a script, see ofn-erase-background here.