Xoops 2.0.7.3 -- groups have stopped working correctly !

I now have 365 users, and XOOPS Groups (Modify) no longer shows the left-hand box for selecting users. Now, when I select Admin > System Admin > Groups > Modify a group, then scroll to the bottom of the page, to the area marked "Edit Members of this Group", I see only one list box, listing only the users that are currently assigned to the group. There is no left-hand box to select users from. Since all my users are registered, the group Registered Users shows 365 entries, and I can delete users from the group, but I can't add new ones !