Thank you for your answer. I checked with several file sizes. Some files of the same, superior or inferior size worked and others didn't, I kept pressing refresh on the FTP client amazed why 2KB files didn't get there and 1MB ones did.
But your tip sent me on the right track. I think the "onComplete" is sent for that temp file you wrote about, so he wasn't waiting for the "move" bit. I checked the upload dir and it was missing the write permission (duh).
I never thought about that because some files got uploaded. But it turned out it was only working for files with the same name, they were already there with write permission and the script overwrote them. It couldn't create new ones but sent that "onComplete" anyway.
Well, it's solved!!! Thanks again for the help.