I now have startet from scratch, including the wipe and installing your rom for gt-n8000 and pico anew.
During the set up I can log in with my nieces account and successfully get through the parental consent.
After I agree to the data protection stuff, I get a screen with "Bleiben Sie hier, um zu erfahren, wie es weitergeht" /"Stay here to find out what's next" and a button that reads "weiter" / "continue". After I click it, it starts searching for some kind of update. This takes a few seconds and then I get an error message, that only tells me to try again later and a button for "cancel". If I click on it, I can start the set up again, but will end with the same error.
I also tried it with creating a kids-account during the set-up, which had the same results.
Edit: I also tried to first set me up as user and than additionally ad my niece as new user, same result (only difference was, it searched longer for this elusive update before the error message.
Edit #2: At "Bleiben Sie hier, um zu erfahren, wie es weitergeht" /"Stay here to find out what's next" I alway have to click "weiter" / "continue" two times. I don't know, if this is already, what derails the process.