I‘d try to split the file. Since a vcf-file is just single vCards stuck together one after the other, this is very easy:
- Open the file in a text editor
- split it anywhere between the lines END:VCARD and BEGIN:VCARD
Another approach is to verify the vcf-file with sabredav.
If that doesn’t help, you could restore the database from your backup and export the addressbooks with the script calcardbackup:
Have a look at the section „does this also work with a broken instance“ in the Readme of the github-repository.