Database entried stopped linking to group interface

I’ve been running JabRef 3.0 without a problem. Now I find for one database (and not the others), the Groups interface is no longer linked to entries. For example, when I click a group, no entries are selected (all entries are greyed out). I can create new links between entries and groups, but old ones are lost. This behavior not found in other databases. The database entries do have a “Groups = {…},” line.

How do I get the Groups interface for this database to respond to old Groups = lines?