Conference Room setup gets "No response from server"

A few of my users are having trouble setting up a conference room with two or more participants. When they pick their Invited Users and then click the Invite button, they get an error message saying “No response from server.” The conference creator is logged in successfully, as are the invitees. And all other Spark functions seem to work fine. Any ideas on trouble-shooting?

Spark = v2.7.5

Openfire = v4.0.3