In the manual I read for EnableAdvancedCatalogingEditor : "This feature does not currently include any support for UNIMARC or NORMARC fixed fields." Is it true? If so, it could be an improvement to make this feature available for UNIMARC installs
Maybe bug 34312 is related?
I think the error message is only about the fixed fields, not about UNIMARC in general. In MARC21 these would be the fixed length fields like 007, 008, etc.
I will try to take a look after this one is fixed https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=32722