Just for reference, and because maybe this will help someone else in the future... here's how you solve this issue:
- Install ipod updater
- Choose "Restore" to erase everything and apply factory settings
- Retry rockbox install
Weirdly enough this solved the issue and worked like a charm here. Rockbox installed flawlessly after this.
Since rockbox was the reason I bought an ipod in the first place... I was kind of scared there for a while.
Thanks for this great piece of code!