Try changing the permissions of the file manager folder and all the subfolders and files to 755. That sometimes works.
Another thing you might try is to:
in the webassist/kfm/index.php page
find the following line:
<script type="text/javascript" src="j/all.php/can-minify"></script>
and change it to:
<script type="text/javascript" src="j/all.php"></script>
Let me know if either of those solve your issue. If not we'll have to open a support ticket to debug it with you directly.