SSL redirect on EC2 instance behind SSL terminating load balancer

I would hazard a guess without you posting your ELB configuration that the ELB is redirecting HTTPS (443/tcp) traffic to the EC2 instance on HTTP (80/tcp). Then you're .htaccess and plugin are trying to redirect it back to HTTPS because it is being seen over HTTP.

Go take a look at your EC2 console under Network & Security > Load Balancers and I would imagine you'll see the Port Configuration says something along the lines of 443 forwarding to 80 (HTTPS, Certificate: blah)

So I assume you have the ELB set to accept traffic on both HTTP/80 and HTTPS/443, but port-forward all to HTTP.

If you want to use your method (which is clever), are you sure what you get isn't %{X-Forwarded-Proto} -- the HTTP: prefix looks odd to me. Other than that, this looks right to me.

If this is in a virtual host or the main server config and it's still not working, you can add

RewriteLog rewrite-log
RewriteLogLevel 3

then look in the file rewrite-log to see what's actually going on. This file can be incredibly verbose, start with lower levels. Logging cannot be initiated from .htaccess files.

(I would suggest you make the flags on the RewriteRule [R=301,L] which will cause the server to send back a 301 response, which tells search engines to consider the redirect permanent, and update its links accordingly.)