Unable to create user with a specific username - Error 504 Gateway Time-out
Platform notice: Server and Data Center only. This article only applies to Atlassian products on the Server and Data Center platforms.
Support for Server* products ended on February 15th 2024. If you are running a Server product, you can visit the Atlassian Server end of support announcement to review your migration options.
*Except Fisheye and Crucible
Summary
Observed HTTP Status code 504 - Gateway Time-out while trying to create a user from the Jira User Management console. 
Environment
Jira Server / Data Center.
Diagnosis
- AddUser.jspa page loads for sometime after clicking on the Create user button and eventually results in error 504
The following warning message appears in atlassian-jira.log:
http-nio-8080-exec-2452 WARN XXXXX 907x4397268x52 xxxxxxx 0:0:0:0:0:0:0:1 /secure/admin/user/AddUser!default.jspa [w.view.taglib.IteratorTag] Value is null! Returning an empty set.
Cause
The error is thrown due to corrupted data in the user data cache and requires manual intervention to resolve it.
Solution
Refresh caches by full Jira restart (all nodes in case of DC): Full restart in Data Center is required because nodes with a corrupted cache will replicate it to other starting nodes, this means shutting down all nodes, validating the service is stopped, and starting them back up, a rolling restart will not resolve this issue.
This feature limitation is tracked in the Feature Request JRASERVER-66095 - Getting issue details... STATUS