[openbox] Openbox context menu not working

[Piscium]

I have been using Openbox for years with Fedora. I upgraded from
Fedora 21 -> Fedora 23 late last year, and since then I have an odd
problem: the Openbox menu stops working some hours or days after
reboot. By "Openbox menu" I mean the menu that appears when I
right-click on some place where there is no window. When this happens,
the scroll wheel stops doing its usual thing which is to switch
workspaces. Alt-Tab still works. Also the mouse works as normal within
a window.

I tried "openbox --restart" (which in the past fixed minor issues for
me) but it has no effect with this particular issue.

I am using Openbox v. 3.6.1.

Any ideas?