Rewrite-question (htaccess)

[AlBundy]

Hello,

I would like to set up a forwarding. (apache server)

this url -> https://www.my-forum.de/kitty-forum/
to here -> https://www.my-forum.de/forums/kitty-forum.21/

i used this:
RewriteRule ^kitty-forum$ https://www.my-forum.de/forums/kitty-forum.21/ [L]

and this:
RewriteCond %{HTTP_HOST} ^my-forum.de/kitty-forum/$ [NC]
RewriteRule ^(.*)$ https://www.my-forum.de/forums/kitty-forum.21/$1 [R=301,L]

But nothing works. :/
Many thanks for the help!