Contact form send no mails when url with trailing slash


I’ve built a simple contact form like this: Email contact form | Kirby CMS

It works BUT when I force the trailing slash via htaccess

# force trailing slash
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !^/api
RewriteCond %{REQUEST_URI} !^/panel
RewriteRule ^(.*[^/])$ /$1/ [L,R=301]

it doesn’t work anymore
No error message - no mail is sent!

What could be the reason?