For starters: Please use example.com in all your examples. If you preview a post, you will see the difference.
Bad news: Your Redirect is doing exactly what you told it to do. mod_alias (Redirect by that name) takes the rest of the path and reattaches it to the changed part.
There are two ways to get your Redirect to do what you want it to do. One is to use a RedirectMatch with some capturing. The other is to use mod_rewrite with the [R=301,L] flag. Both ways, you will need to change the syntax a little bit.
Does your htaccess or config file currently use both mod_alias (Redirect or RedirectMatch) and mod_rewrite (RewriteRule)? If yes, change everything to mod_rewrite for safety. If no, use a RewriteMatch.
The 403 that you are seeing is not a "pure" 403. It is the result of an infinite loop: a series of redirects that end up at 403 because the server stops after 10 rounds of doing the same thing. Stop and study your original redirect and you will understand why this is happening.