Does haproxy cache content by default

What are you using to cipher/decipher the SSL traffic before haproxy ? Stunnel, nginx, apache, something else ?

I suspect it might be related to the lack of "option httpclose" on your port 80, but it's not clear to me why it would cause an issue to only a few visitors.

Remove the check port 9200s then your backup option should work. You seem to be mixing different examples, the mysql-check works on the standard port wheares there is another common example that uses an http check where they setup a xinetd process answering on port 9200 which runs a seperate check script.