I can't register my SimpleLeak because CRA rejected the modem ownership transfer

What can I do if the SimpleLeak end product certificate doesn't work?

If you encounter this:

This is okay. Even though the SimpleLeak has end product certificates for both RC1 and RC3, devices from a certain batch cannot be registered with this certificate (there was a misunderstanding with Sigfox). To register your devices, please follow this guide: https://ask.simplehw.eu/register-as-a-prototype