When I type letters "m", "c" or "e" in user field name or assignee, Jira send me the next error " The JIRA server was contacted but has returned an error response. We are unsure of the result of this operation." I recently changed email adress from some users.
I´ve Jira server 7.2.2
I believe this is an issue with your version. We had a similar issue on 7.2.1. I would recommend upgrading:
If this is not an option, try different browsers. I found it happened in Chrome and not IE.
Another trick is to copy and paste first names into the user picker field. In my experience, the issue mostly had to do with looking up users by just one letter but more than one character didn't throw the error.
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