From Jira Project Settings --> "Users and roles" side --> "Add users to a role" button, when we start typing for example C, E, T (and more characters) it is getting the following error :
"The Jira server was contacted but has returned an error response. We are unsure of the result of this operation."
The following bug issue was opened on 21/Dec/2017 to keep track of this problem and its status is still Gathering Impact.
Could you please click on Affected customers as "This affects my team"? from the following link?
Maybe Atlassian will solve this issue soon.
Thanks for your time.