Okay, I didn't see the solution in the FAQ section, but I thought I'd have a look around. My German is poor, so I was intimidated at first. Seems this one has been solved elsewhere (multiple times).
On some servers, where the folders/categories are created by a process and thus an ID (48 in this case), other process are having a problem writing to those folders.
An easy solution is to delete the folders in your FTP client (1,2,3,4 etc.) and then re-create them while in your superuser account. You'll see the owner is no longer "48" but "<username>". Don't forget to set permissions to 777 on each folder. You'll need to repeat these steps for both the media and thumbnails directories.
Then everything works like a charm.
Here is a link to another post on the subject for more background:
http://www.4homepages.de/forum/viewtopic.php?t=3704&start=0&postdays=0&postorder=asc&highlight=safe%20mode
