It came from the battery. See photo (it is not that clear but something is missing there)... so it is the micro-B USB connector. I will contact support@petoi.com. Much appreciated!

I am not a specialist in batteries but those batteries have the wrong voltage (1.2V instead of the preferred/required 3.7V) and form factor (AA instead of the slightly larger 14500 - this one is not required but a good indicator if you have the right batteries)! I have NiteCore NL1485 850mAh 14500 3.7V 3.15Wh batteries. Try shopping in specialized battery shops but I bought mine in bol.com (living in Belgium). (For the other questions, I haven't painted my Nibble (yet?) and although I bought a Raspberry Pi 4B and connected it, I have not experimented yet with it... took a break when I finally saw Nibble walking, yeah! Next vacation is make Nibble think...)

Hey everyone, I think I am living in the worst tax country in the world: Belgium - that tiny country in Europe. Not only customs that took more than a month. I had to pay 79,45 EURO. That is 21% VAT and 30 EURO customs fee (WTF?). Cheers, Christian.