Single journal url .htaccess configuration

Dear @ctgraham

the issue is as here: SOLVED: Upgrade from OJS 3.1.2.4 to OJS 3.2.1.2: problems with url api in back-office
or here: [OJS 3.2.0.3] Error on Submission Page - #22 by drugurkocak

base_url = "https://www.medienpaed.com"
base_url[medienpaed] = https://www.medienpaed.com
restful_urls = On

I pretty much tried all of those .htaccess rewrite rules but none really work. They either 404 or 500 for the whole site or they don’t properly affect the API calls. My current .htaccess is (from: [Solved] Problems with api submissions error on submissions page - #10 by ctgraham):

# If the URI already has index.php in it, don't change it
RewriteCond %{REQUEST_URI} !/index.php/
RewriteCond %{REQUEST_URI} !/medienpaed/
# Skip existing directories if Apache 2.2 or later
#RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
# Skip existing files if Apache 2.2 or later
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
# Rewrite all other requests to a specific journal in OJS
RewriteRule ^(.*)$ index.php/medienpaed/$1 [QSA,L]

# If the URI already has index.php in it, don't change it
RewriteCond %{REQUEST_URI} !/index.php/
RewriteCond %{REQUEST_URI} /medienpaed/
# Skip existing directories if Apache 2.2 or later
#RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
# Skip existing files if Apache 2.2 or later
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
# Rewrite all other requests to a specific journal in OJS
RewriteRule ^medienpaed(.*)$ index.php/medienpaed/$1 [QSA,L]

Too bad that this code only removes the error message and puts the API request in some loop, without any effect (bonkers?). Thus I can’t use filters on submissions etc.
As earlier posts mentioned that redirect in site table should make redirecting single journal instances very easy, I started to try to figure out what this actually does. Its value used to be set to "1". A fresh install sets or leaves the value on “0”. But changing the value has no effect.

Any idea?