(1) I can confirm the issue also for 'icewm', 'wmaker', 'wmii', and 'xfwm4'. The latter two seem to behave slightly differently (e.g. not showing anything), but the impact is the same (i.e. no 'wbarconf' window to be available for the user).
(2) I've also done a quick test with 'fluxbox', 'hackedbox', 'evilwm', 'tinywm', 'windowlab', 'aewm++', 'flwm', 'jwm', and 'i3'. And those WM appear to work just fine.
(3) The source file available (i.e. 'wbarcanf.fl') had already been updated, so I had no chance to see what has changed in the code.