Slave running Debian 7 can not be started

Since a recent update (to 2.58) my slave jenkins node will not launch.
The slave runs debian 7 with oracle jre 1.8.0_131.
The master runs debian 8 with the same jvm.
Launching the slave fails on the first ssh connection the master tries to make.
In the /var/log/jenkins/jenkins.log on the master:

[05/01/17 08:09:14] SSH Launch of OpenEmbedded Buildserver on mySlave failed in 4,899 ms
May 01, 2017 8:09:14 AM hudson.remoting.SynchronousCommandTransport$ReaderThread run
SEVERE: I/O error in channel OpenEmbedded Buildserver
java.io.IOException: Unexpected termination of the channel
at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:73)
Caused by: java.io.EOFException
at java.io.ObjectInputStream$PeekInputStream.readFully(ObjectInputStream.java:2335)
at java.io.ObjectInputStream$BlockDataInputStream.readShort(ObjectInputStream.java:2804)
at java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:802)
at java.io.ObjectInputStream.<init>(ObjectInputStream.java:299)
at hudson.remoting.ObjectInputStreamEx.<init>(ObjectInputStreamEx.java:48)
at hudson.remoting.AbstractSynchronousByteArrayCommandTransport.read(AbstractSynchronousByteArrayCommandTransport.java:34)
at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:59)

On the slave in /var/log/auth.log:

May  1 14:03:27 nvs0559 sshd[31795]: fatal: dh_gen_key: group too small: 1024 (2*need 1024) [preauth]

Upgrading the slave to debian 8 fixes it, but I really need debian 7 for the project to be able to build it.