Redirect POST htaccess
Your "add trailing slash" rule forces a header redirect:
[R=301,L]
a header redirect will drop POST values.
You will have to drop that rule, or disable it for POST submissions:
# Forces a trailing slash to be added
RewriteCond %{REQUEST_METHOD} !=POST
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(\.[a-zA-Z0-9]{1,5}|/)$
RewriteRule (.*)$ /$1/ [R=301,L]