@Null I'm sure more competent people have already reached out to you, but
Timeout during SSL handshake just means that the communication between the server & the load balancer timed out. At least I'm assuming that your connections between the LB/Haproxy and the content servers are SSL encrypted. Generally the timeout is sixty seconds or thereabouts, so assume that whatever the attack is doing it's slowing down the responses enough that the response from the content servers is >$howevermanyseconds. SSL is just the first thing to croak, so you're seeing it in the logs.