memento17 on "Media Uploads doen't work after moving WordPress"

I realy hope that somebody can help me out.
I copied my wordpress-site to a subdirectory on the same server to "experiment" with this copy before editing something on my "real" site. So far so good. The only problem left: the media upload does not work on the copied site in the subfolder. I get an

Unable to create directory wp-content/uploads/2016/05. Is its parent directory writable by the server?

I checked and my wp-content folder and the uploads folder have a 755 permission.

I thought I might update the path to the directory in wich the media should be uploaded, since it's not possible in the backend in my version (4.2.1) I tried to set a path in my wp-config with

define( 'UPLOADS', 'wp-content/'.'uploads');
or

define( 'UPLOADS', 'mysubfolder/wp-content/'.'uploads');

but that didn't work for me. Than I tried to create the 2016 and the 05 folder manually, but still the same error...

In my "old" installation, which is running with the same WP-Version everything works fine. So I have no Idea what else I could try. Every Idea or help is appreciated!