It's been a while since this was reported as 'implemented'.
However, I experience the exact same issue.
Web -> Contacts -> Right click on Contacts -> Import.....
It's a multi-contact *.vcf file.
Only 1st contact is imported.
I manually add a contact, save it.
I export the contact to a *.vcf file (single contact here)
I delete the contact from my Contacts
Import that just exported contact.
Import Error: Unable to import "Ops_COPCC.vcf". The file is not valid.
So grommunio contacts can not also import its own exported contacts.
grommunio Web: 126.96.36.1992819a-lp154.4.2