Update: This has been fixed in Rebex FTP/SSL 2015 R4.
Thanks for the logs, now I can see that this is an already known bug, to fix it please download the trial hotfix and give it a try. The fix will be part of next release, i.e. 2015R4.
Customers with active support contract can request the full version of the hotfix at support@rebex.net based on their order ID or registered email.