Karaf Console doesn't open

Karaf console doesn’t open anymore. I have build 1611 on a Raspberry 3b+ with an up to date Raspbian. It has worked some times ago but now it closes immediately after launch. I have tried:

openhab-cli console
ssh -p 8101 openhab@localhost

Always the same result. In the Log I see:

[WARN ] [shd.server.session.ServerSessionImpl] - exceptionCaught(ServerSessionImpl[openhab@/])[state=Opened] RuntimeSshException: null

Any ideas what’s wrong?