Same problem for me if I try to enable Compiz plugins by using DBus!
This is how to reproduce my bug: simply launch this command:
   $ dbus-send --type=method_call --dest=org.freedesktop.compiz /org/freedesktop/compiz/core/screen0/active_plugins org.freedesktop.compiz.set array:string:'dbus','decoration','place' 

And compiz says: 
 > compiz (core) - Warn: Can't set Value with type 12 to option "active_plugins" with type 11 

I think I use the right command because this is the example written in the code there: http://git.compiz.org/compiz/core/tree/plugins/dbus/src/dbus.cpp (I've replaced 'allscreens' by 'screen0')
Or do I use a wrong command? 
Compiz gives me the same error if I use this function (in C):

    dbus_g_proxy_call_no_reply (proxy,
    			"set",
    			G_TYPE_STRV,
    			plugins, // type= **gchar
    			G_TYPE_INVALID)

Note that I've no problem to receive the list in C or with this command:
   $ dbus-send --type=method_call --print-reply --dest=org.freedesktop.compiz /org/freedesktop/compiz/core/screen0/active_plugins org.freedesktop.compiz.get

PS: please have a look to these commands and the output messages:
   $ dbus-send --type=method_call --print-reply --dest=org.freedesktop.compiz /org/freedesktop/compiz/core/screen0/active_plugins org.freedesktop.compiz.get
   method return sender=:1.36 -> dest=:1.138 reply_serial=2
   array [
      string "core"
      string "ccp"
      string "bailer"
      string "crashhandler"
      string "detection"
      string "composite"
      string "opengl"
      string "copytex"
      string "imgjpeg"
      string "compiztoolbox"
      string "decor"
      string "firepaint"
      string "gnomecompat"
      string "mousepoll"
      string "place"
      string "text"
      string "grid"
      string "water"
      string "dbus"
      string "imgsvg"
      string "commands"
      string "move"
      string "regex"
      string "resize"
      string "imgpng"
      string "wall"
      string "vpswitch"
      string "ring"
      string "blur"
      string "thumbnail"
      string "session"
      string "animation"
      string "animationaddon"
      string "loginout"
      string "obs"
      string "wobbly"
      string "fade"
      string "expo"
      string "workarounds"
      string "ezoom"
      string "scale"
      string "scaleaddon"
      string "widget"
   ]

   $ dbus-send --type=method_call --print-reply --dest=org.freedesktop.compiz /org/freedesktop/compiz/core/screen0/active_plugins org.freedesktop.compiz.set array:string:'dbus','decoration','place'
   method return sender=:1.36 -> dest=:1.139 reply_serial=2

   $ dbus-send --type=method_call --print-reply --dest=org.freedesktop.compiz /org/freedesktop/compiz/core/screen0/active_plugins org.freedesktop.compiz.get                                         
   method return sender=:1.36 -> dest=:1.140 reply_serial=2
   array [
      string "core"
      string "ccp"
      string "bailer"
      string "crashhandler"
      string "detection"
      string "composite"
      string "opengl"
      string "copytex"
      string "imgjpeg"
      string "compiztoolbox"
      string "decor"
      string "firepaint"
      string "gnomecompat"
      string "mousepoll"
      string "place"
      string "text"
      string "grid"
      string "water"
      string "dbus"
      string "imgsvg"
      string "commands"
      string "move"
      string "regex"
      string "resize"
      string "imgpng"
      string "wall"
      string "vpswitch"
      string "ring"
      string "blur"
      string "thumbnail"
      string "session"
      string "animation"
      string "animationaddon"
      string "loginout"
      string "obs"
      string "wobbly"
      string "fade"
      string "expo"
      string "workarounds"
      string "ezoom"
      string "scale"
      string "scaleaddon"
      string "widget"
      string "dbus"
      string "decoration"
      string "place"
   ]

=> 'dbus', 'decoration' and 'place' have been added to the list but nothing has changed in Compiz and this is not what I want (I want to replace the list)