When clicking on "Assign" button of an issue and typing to find a user, the search finds No matches for certain users. For others, it works.

No errors are displayed on the logs and I was only able to replicate this behavior on Firefox 3.6.28, on the latest versions, it works fine. Also, not able to replicate on Standalone.

Also, all the permissions were okay. If you edit the issue, you are able to change the assignee by editing the assignee field as well.