Server returns an 'error response' when attempting to set an assignee when using the 'Create Ticket' modal

When attempting to set an assignee on the 'Create Ticket' modal, I receive the following error:

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

When I dismiss the message, I am able to use the type ahead to set an assignee and everything works correctly.