By importing some vCard files (.vcf), the import function ends with “Input string was not in a correct format” and aborts. The same vCard files are correctly opened by other email software.
What program are you exporting to vCard from ?
I’ve used an OST to vCard converter. All vCards saved this way are opened by Windows Mail, Thunderbird etc
I suggest you open it in one of those applications, and then export it from there before importing it into eM Client. There may be something added by the converter that is not compatible with eM Client.