Did you check the documentation and did you create the document with the known hosts and private key. You mentioned those at the end of your post, but did not state if you created the documents with the data.
Regards,
Ronald
Are you the application in the bussinesmodeler or in a hosting environment?
In case of the modeler: Is emulate cloud security enabled? If it is you can try to modify your policy file in the runtime folder of your business modeler. Search this forum for policy file and find some instructions how to add
java.net.SocketPermission ftp.colimbra.net resolve