Openfire 4.1.3, all online contacts intermittently disappear

I have installed Openfire 4.1.3 on a Linux server and am connecting with Spark 2.8.3. I have LDAP configured, and everything works well most of the time. Periodically, however, my list of users on Spark gets emptied: Groups show, but each group is empty. Usually, if I log out and log back in, that fixes the problem. While my contacts are empty, other people can apparently see me and contact me. I think this happens with other uses as well. And it happened on our previous installation (I believe it was 4.1.1).

Is this likely a bug, or might I be missing something else?