The connection with slave via ssh :

Launch slave agent via SSH, Know hosts file Verification Strategy

Slave log:

[03/12/18 10:58:19] [SSH] Opening SSH connection to xxx:22.
[03/12/18 10:58:19] [SSH] SSH host key matches key in Known Hosts file. Connection will be allowed.
[03/12/18 10:58:19] [SSH] Authentication successful.
[03/12/18 10:58:19] [SSH] The remote user's environment is: ...

Expanded the channel window size to 4MB
[03/12/18 10:58:19] [SSH] Starting slave process: cd "/home/dev/jenkins-smals-slave-3" && java  -jar slave.jar
<===[JENKINS REMOTING CAPACITY]===>channel started
Remoting version: 3.14
This is a Unix agent

But slave is still OFFLINE and the button 'Relaunch agent' is visible.

The connection via terminal ssh is OK.

Delete slave and atach them again in Jenkins did not work for us.

We are really blocked now.