big trouble with redirecting

i’ve created successfully a network site based on subfolders (/en/ and /de/)
then i just wanted that all visitors will see the /en/ version instead of /
so i replaces my preview index.html on the root with a index.php with a redirect to /en/

but then i am facing big trouble that my site /en/ is not showing – instead a 404 page.
so i removed the index.php and use again the index.html as a preview.
did not solve my problem.

HELP

i cleared all cashes and cookies and uses different browsers.

https://opensmjle.com/en/
strangely i can still access /en/wp-admin/

i have turned on wordpress debug mode. got these two errors (maybe they have been there before)

Notice: load_plugin_textdomain was called with an argument that is deprecated since version 2.7 with no alternative available. in /usr/www/users/opensmk/wp-includes/functions.php on line 2925

Warning: Cannot modify header information - headers already sent by (output started at /usr/www/users/opensmk/wp-includes/functions.php:2925) in /usr/www/users/opensmk/wp-includes/pluggable.php on line 876

thanks
oli

ps
my .htaccess file:

## redirect all to opensmjle.com without www
RewriteEngine On
RewriteCond %{HTTP_HOST} ^www\.opensmjle\.com [NC]
RewriteRule ^(.*)$ https://opensmjle\.com/$1 [L,R=301]

RewriteCond %{HTTP_HOST} ^www\.opensmjle\.de [NC]
RewriteRule ^(.*)$ https://opensmjle\.com/$1 [L,R=301]

RewriteCond %{HTTP_HOST} ^www\.opensmile\.com [NC]
RewriteRule ^(.*)$ https://opensmjle\.com/$1 [L,R=301]

RewriteCond %{HTTP_HOST} ^www\.smjle\.de [NC]
RewriteRule ^(.*)$ https://opensmjle\.com/$1 [L,R=301]

#redirect imprint
RewriteRule ^imprint[/]?$ https://opensmjle.com/en/ [R=301,L]

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]