It was not the cause. Sorry for not being complete, but I really didn't think this had anything to do with it: I was testing first if the file was complete with
flock($f, LOCK_EX) or return; #file in use
apparently I must do a
flock($f, LOCK_UN);
although the transfer was in the same procedure, one line below. I don't understand why..
| [reply] [d/l] [select] |