After github.com crypto update here, jgitflow is not working anymore using git:// (ssh). There is only one workaround to rely on, via https protocol. (TLS 1.2). Let's agreed that if nobody take care of this issue, this project must be considered as un-supported.
The -X flag show the issue in jsch, It should be nice if someone update to 1.54 to see if that should fix the pb.