Mailing List “Search All” no longer works

In Marketing > Email Campaigns > Mailing Lists, the "Search All” tool is no longer working.

It's not a matter of individual emails or opt-in permissions. Not one email address or contact name will appear—including my own. Every search on each of my four mailing lists produces "No results.”

I've tried logging out and back in. No luck. It used to work just fine. Now, not at all. 

The names and addresses are indeed in my Contacts list.

Any suggestion?