Thanks Frédéric!
Are you sure you haven't for example done a MARC21 installation and then switched to UNIMARC via Koha system preferences? In this case, your koha-conf.xml file points to MARC21 Zebra configuration files, and your Koha instance send to Zebra UNIMARC records. You can see if it is the case in this config file:
That was it! I followed to wiki pages to install koha from the apt repository, and they say to use 'koha-create' to create your site. Nowhere they mention marc21/unimarc, or anything... Following your suggestion above, I looked at koha-conf.xml, and found that my system was not unimarc, but my database and koha were unimarc. I'll try creating a new site, and this time add a "--marc-flavor unimarc" to my command. :-) Thanks!