Cannot mass import users/members

I'm trying to mass import user data from a CSV using WP All Import (paid for full version). Every time I try and import, I get a few records in and then "Import Error - Registration Date is empty". Any ideas what this could mean?

We have a number of custom fields but this doesn't seem to be the issue. I've also checked WP All Import default troubleshooting steps and there aren't any issues there either (I've run successful imports on this site before for other data).