Unable to add users as Jira software board admin via board settings for migrated data from Cloud. Error: "Ug" is not a valid user

Summary

After recently migrating from Jira Cloud to Server or Data Center, unable to add new users as Jira software board administrator via board settings, the following error is thrown:

Error:
"Ug" is not a valid user.

This is caused by the data set introduced in Cloud: https://jira.atlassian.com/browse/JSWCLOUD-20732

Steps to Reproduce

• Have a Cloud instance with users created around Sep~Oct 2020;
• Export an XML Backup from this instance and migrate to Server
• Try to add a new user as a board admin in any Jira software board;
• Jira will throw "Ug" is not a valid user. error
• Querying the app_user table looking for that user lower_user_name we should find the following:

  ID	user_key	lower_user_name
  11054	ug:1287e681-a24c-44ec-bac7-e72ad966a9a2	psouza

Expected Results

Jira should either convert those user_keys to the Data Center pattern during the export/import, or have the same bug fix implemented that will work with those 'ug:' user_keys.

Actual Results

The following error message is seen upon adding a user:

Workaround

No workaround available. Please upgrade to the fixed version(s) as listed above.