just for information if anybody runs in same problem. when i installed the Megamenu i got a blank page in the admin area when i tried to add a new Megamenu. Problem solved this way:

in the file categories.phtml at line 28 is the bug, it has \ instead of / (for file path !)

instead of

$path = $mediaPath.'code\Netbaseteam\Megamenu\view\adminhtml\web\html\config.json';

it must be

$path = $mediaPath.'code/Netbaseteam/Megamenu/view/adminhtml/web/html/config.json';