There was such an issue, and it was closed with the answer that the person should follow some Python rule and change locale settings to explicitly use UTF-8. That’s weird. So Debian is only for English-speaking people?
I changed to:
export LC_CTYPE=C.UTF-8
export PYTHONIOENCODING=utf-8:surrogateescape
export LC_ALL=C.UTF-8
export LANG=C.UTF-8
in /etc/profile, deleted .homeassistant and performed a complete reinstallation
But the error persists. I still get e.g. characters 0xE5 and 0xF6 in some json file the second time I try to start homeassistant.