Sorry to resurrect such an old thread, but I'm having the exact same issue now on FreeBSD 13-BETA2.
[Mod: post split off from https://forums.freebsd.org/threads/window-maker-could-not-exec-wprefs.65622/]
[Mod: post split off from https://forums.freebsd.org/threads/window-maker-could-not-exec-wprefs.65622/]