Solution:
My contacts were still on a Moto Android so exported the lot, 689 contacts, to a .vcf.
Upon importing the file into to Contacts app, 216 contacts were not imported.
I moved the not-imported contacts to separate .vcf and tried to import again and got the dreaded "no importable cards were found" :-(
Much tried and test ensued.
It turned out that the first card of the not-imported contacts file had the "NOTE" record split in three lines.
Turning the three lines into a single one by deleting linefeeds (and extra "=" characters), solved the problem and all remaining records were imported.
Hope it helps.