You cannot mix and load balance between HTTP and HTTPS.
Separate them.
defaults
mode tcp
frontend http
bind 192.168.0.100:81
default_backend httpbk
backend httpbk
server s2 192.168.0.100:80 check maxconn 20
frontend https
bind 192.168.0.100:4043
default_backend httpsbk
backend httpsbk
server s1 192.168.0.100:443 check maxconn 20