I was having problems connecting ubuntu with protostar vmbox. Accomplished it running this commad:
ssh -v -oHostKeyAlgorithms=+ssh-rsa -p 2222 [email protected]
With this network config in the vmbox:
Name: SSH
Protocol: TCP
Host IP: 127.0.0.1
Host Port: 2222
Guest IP: 10.0.2.15
Guest Port: 22