The problem happened several times, but I am not able to reproduce it anymore. I have logging enabled now, next time the problem occurs I'll get the log and reopen this case.
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
The remote system is a Unix system, but I do not know which manufacturer/OS version. Protocol in use is SFTP (v3). I hope this info is enough.