Error when importing test cases using Zephyr Importer utility tool

We are on: Zephyr for Jira 6.0.1 - Test Management for JIRA (Server)

and Jira v8.13.4

We were using the Zephyr Internal Importer successfully following the instructions on the Zephyr doc at this page https://zephyrdocs.atlassian.net/wiki/spaces/ZFJ0500/pages/1061421425/Import+Tests starting in the "Using the Internal Importer" section. But it has stopped working and now when we get to the last step of clicking the "Import" button we get an error.

We have tried to reformat the excel file being imported in multiple ways but nothing is working.

The error on screen says it was successful but also that it errored - and no test cases are actually imported. "Error: Sheet1:Row:2:null". I've pasted a screenshot below.

The logs also show errors like this (where I've replaced the person's real user name with 'xxxuser':

2021-03-10 13:45:57,892 pool-8077-thread-1 ERROR xxxuser 825x6044494x1 g1bsm5 10.217.229.68,172.27.70.88 /rest/zephyr/latest/importer/issueImport [c.t.z.j.service.impl.ExcelFileProcessThread] Exception while creating issue: