I'll better du the same then, copying my answer from the issue :-)
Thanks, but there doesn't seem to be any null character in the input file. I have tried several things:
First I tried tr < file-with-nulls -d '\000' > /opt/mailman-web-data/tmp/${LIST}.mbox and a docker exec -it mailman-web python manage.py hyperkitty_import --verbosity 3 --since "01.01.1970" --list-address ${LIST} /opt/mailman-web-data/tmp/${LIST}.mbox (with $LIST being the name of the list).
Then I tried to vim -u NONE -U NONE -c 'set hls dy=uhex' input file, searching for NULL-characters (ctrl-v 000<enter).
I have tried sed -n '/\x0/p' input file and grep -Pa '\x00' input file and other commands that should show me any occurrences of null characters.
None of them indicates any null characters in the file that hyperkitty_import complains about.