症状

1. The Assignee and Reporter fields do not find users in the auto-complete drop-down
2. Attempting to manually assign a user results in an error: "Assignee: Failed to set field"

原因

The user does not have Browse users and groups global permission which is required to find users in the auto-complete drop-down.

ソリューション

1. Go to the Jira Administration Console > System > Global Permissions

2. Ensure that the necessary groups are granted to the Browse users and groups global permission