JIRA import and convert to Atlassian Account from portal only customers failing with "failed to import user remotely" error message

Summary

JIRA import and convert to Atlassian Accoount from portal only customers fails with the error message "Failed to import user remotely" if a user has a special character (German umlaut for example) in their Full Name.

Steps to Reproduce

1. Import a JIRA backup from an instance where a user has special characters in their full name.

And

1. Convert to Atlassian Account from the customer portal only user that has special character inside their Full Name.

Expected Results

The import is successful and all users are created.

Actual Results

Import fails with the error message "Failed to import user remotely" and the following error is displayed in JIRA Logs:

@400000005874faab25a43524 Caused by: com.atlassian.idp.client.exceptions.ServerException: {"errors":[{"message":"Invalid UTF-8 middle byte 0x3f\n at [Source: org.apache.catalina.connector.CoyoteInputStream@53c43a0d; line: 1, column: 187] (through reference chain: com.atlassian.idp.model.Json[\"displayName\"])","isHtml":false,"path":null,"fieldIndex":null}]}

Workaround

Remove all special characters from Full Name, generate a new backup, and use this new backup to perform the import.

Notes

Please note that you may encounter this error message as well if you have more than 1 user accounts using the same email address (even if the user account is deactivated). Steps are the same except you will need to update the email address of one of these users, then generate a new backup, and import it.