I've had this same problem, but found a work around for it (until it is fixed).
The problem appears to be in "webassist/kfm/configuration.php", approximately line 29. (it's line 29 on my system, but I guess that could vary)
$kfm_userfiles_address = '../../path/to/folder';
The above line should include a closing "/" (../../path/to/folder/)
I can manually code that slash in, however if I were to edit the file manager uploads from the HTML Editor server behaviour, it strips the closing slash out and I have to put it back in again manually.
It's a quick update on one file for me so I'm not worried about it, but would like it fixed :)