I think it's more easier to have an eye on the screenshot in this report.

The problem,

When you have more than 10 folder in a folder witch everyone contain subfolder, and if the second subfolder contain a subsubfolder, in the web mail when you want to move a mail into this folder it's impossible. In the interface you will see the eleventh subfolder in the list. (Not the good one)

When i have a look to the generated code, the popupmenu are build like this
<div id="move-00010Submenu" .....>
<div id="move-00011Submenu" ...
The fourth number with "move-" describe the position of the folder in the subfolder.
The fifth number describe the position in the subsubfolder.
If you have an eleventh folder, there is noway to make difference between the eleventh folder or the second subsubfolder.

I hope my english is quite enought for fixing this bug.