This can be caused by a number of things. Are you now running Wowza Streaming Engine 4.3 with the included JRE (Java Runtime Environment)? If not, or if you are running your own Java VM then I'd suggest installing the latest version 8 JRE (or JDK if that is a requirement).
You can explicitly state the size of the DH Key by adding the following VMOption in your [install-dir]/conf/Tune.xml file (restart Wowza once done).
If this does not resolve matters then you may need to generate a new StreamLock certificate, which you can do via the Wowza Portal, in the StreamLock tab.
If this still does not resolve the matter, then I'd suggest enabling SSL debug and raising a support ticket. Please refer to this thread and include the information requested in the raising a ticket page.