Error when choosing down arrow on Assignee field

Summary

When assigning an issue to a valid assignee in JIRA Cloud, this error message occurs:

"The JIRA server was contacted but has returned an error response. We are unsure of the result of this operation."

Environment

JIRA Cloud 6.5-OD-08

Steps to Reproduce

Click Create > select the drop down, the error message will occur.

This seems to be happening with all Projects.

Expected Results

The drop down menu should populate with assignees

Actual Results

The below exception is thrown in the xxxxxxx.log file:

"The JIRA server was contacted but has returned an error response. We are unsure of the result of this operation."