For anyone trying this jailbreak on firmware => 5.16.3, make sure you're installing the hard float version of MRPI (MR Package Installer) after setting up WinterBreak and the universal hotfix.
PS. KOReader current nightly builds for kindle hard float firmware (named koreader-kindlehf) may not work for everyone, the KOReader team is working on it. If this is your case, try Marek's nightly builds in the meantime.
PSS. The KOReader links above only work on firmware => 5.16.3. If you're running anything lower than that, you can download from the KOReader releases page
Read the kindlemodding.github.io wiki for newer instructions.