Atlassian Stash default server setting for SSH is port 7999. If you switch the git repository to SSH access, your client may reject the SSH connection.
To resolve this try to login with ssh to the host
On Windows with plink (Putty Link)
$ "C:Program Files (x86)PuTTYplink.exe" gitstash -P 7999 The server's host key is not cached in the registry. You have no guarantee that the server is the computer you think it is. The server's rsa2 key fingerprint is: ssh-rsa 2048 .. If you trust this host, enter "y" to add the key to PuTTY's cache and carry on connecting. If you want to carry on connecting just once, without adding the key to the cache, enter "n". If you do not trust this host, press Return to abandon the connection. Store key in cache? (y/n) y