Creation of multiple folders (when copying data from my system to box server) fails!

Hi

I get an error message “Ein oder mehrere untergeordnete Ordner konnten nicht hochgeladen werden.” (some or more Subfolders could not be uploaded).

What is wrong?

I just drag and drop my folder that is on an external drive into the application window, where all data is “stored” and listed.

How to solve this?

Thanks, 

Matt