@tredelijkheid said in Trying to add a new system - stuck already with the system.svg...:

someone could look at the file and tell me what is wrong. Again, it looks quite OK but in ES it does not.

I quickly checked the file and it seems ok, as you said.
I noticed the image "size" (dimension of the page in inkscape) is quite big compared to usual system svgs. ES does scale it but sometimes that scaling has issues.
I would suggest you open one of the existing ES system.svgs (or any of my custom system icons) , save it with a different name, delete all objects (making it "empty"), open your sinclair svg, select all objects and paste them in the "empy" svg, group all objects and save.
That should fix your problem.