Hi and thanks for sending a fast reply. At first I want to tell you that I am not from the same organization as En-Tie, I only wrote to this thread because I can see that we are sharing exactly the same problem within similar environments. This is what the Upload Center looks like, "upload all" nor "Resolve" doesn't help.
I hereby confirm that I am using SharePoint through the Office 365 (P1).
-ThomasR