When importing data from Trello, multiple errors are shown related to users

Issue Summary

When using the External System Import, selecting the Trello option, to import data from Trello, multiple errors occur related to user import.

Steps to Reproduce

1. Create a Board in Trello;
2. Create some lists with some cards on it;
3. Add users to these cards, come users can belong to a Jira instance, and some don't;
4. Go to External System Import in Jira, select the Trello option;
5. Follow the instructions to import the desired Board;

Expected Results

The import result with no errors.

Actual Results

Depending on the case, some errors are thrown. A couple of examples:

• If the user already exists in the Jira instance:
  Cannot create user X: User X with status InactiveDueToLicenseRolesEnabled was not found after creating them.
• If the user doesn't belong to the instance:
  Cannot create user Y: java.util.ConcurrentModificationException: com.atlassian.jira.crowd.embedded.ofbiz.UserNotFoundException: User Y does not exist
• Some users might lose their current permissions on the Jira instance, overwritten by the Trello import.

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available