The jgitflow-maven-plugin is not maintained anymore, the last commit was made 2015. There are some issues that are open and most likely will not be fixed, espacially the out-dated TLS / SSH support.
Due to this bug (https://ecosystem.atlassian.net/browse/MJF-301) it's no longer possible to use a Github Repository with SSH but you need to fallback to https.
I'd recomment to use the actively maintained gitflow-maven-plugin instead. It provides the required configurations currently used and some additional ones (e.g. automatic push of the changed branches & tags).
What are your thoughts ?