Thanks for the replies.
The tracking information did contain an Error to be more precise a Gateway Timeout, just as Ben Cline predicted.
The ftp log shows that the whole file, the file being 4701209 bytes , was uploaded:
2012-04-18 06:18:50 56928 DataChannelOpened - - 0 0 0 0
2012-04-18 06:18:54 56928 DataChannelClosed - - 0 0 4791209 0
Is there any way to work around this problem?
Thank you all in advance,
Mathias