I hate to dig this up out of the graveyard, but I am having a very similar issue. I have a slideshow on the same page as with the Layers Menu. After the slideshow loads the next image, I get "'numl' is undefinded". I have traced it back to the phplayersmenu.
I have found this issue in both IE 6 and IE 7. Firefox doesn't seem to record any errors that I can see of.
You can see it for yourself:
This is just a test gallery:
I can provide more information, just let me know what you need.
I forgot to mention, I am using PHP Layers Menu 3.20-rc.