Does this happen regularly, or intermittently? It looks like some kind of SSL issue from the stacktrace, but I'm afraid I don't have any experience with this configuration.
Another troubleshooting approach would be removing components to see if that changed the behavior.
Or you could also search for interactions between Hikari (the java connection pooling system FusionAuth uses) and haproxy.
Sorry I don't have a better answer for you. Please let us know what you end up finding out!