try to change this:

#RewriteBase /selfoss

into

RewriteBase /rss

Greets
Mario

You need to check your .htaccess file.

Not to be a jerk or anything. But that does not help me in any way.

Maybe you can give me an example of what I'm looking for or what I can add?
Here is the content of my .htaccess file:

#AddHandler php53-cgi .php

# Enable rewrite engine and route requests to framework
RewriteEngine On
#RewriteBase /selfoss

# Index page but don't list on search engines
Header set X-Robots-Tag "noindex, nofollow"

# rule for favicons
RewriteRule !data/favicons/(.*)$ - [C]
RewriteRule favicons/(.*)$ data/favicons/$1 [L]

# rule for thumbnails
RewriteRule !data/thumbnails/(.*)$ - [C]
RewriteRule thumbnails/(.*)$ data/thumbnails/$1 [L]

# rule for front controller
RewriteCond %{QUERY_STRING} ^(.*)$
RewriteRule !.(js|ico|gif|jpg|png|css|asc|txt|eot|woff|ttf|ttf|svg)$ index.php?%1

# rule for files
RewriteRule !favicons/ - [C]
RewriteRule !thumbnails - [C]
RewriteRule !public - [C]
RewriteRule !index.php - [C]
RewriteRule (.*) public/$1 [L]

# deny requests for config files
<FilesMatch ".(ini)$">
Order allow,deny
Deny from all
</FilesMatch>

# Disable ETags
<IfModule mod_headers.c>
Header Unset ETag
FileETag none
</IfModule>

# Default expires header if none specified (stay in browser cache for 7 days)
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault A604800

ExpiresByType application/javascript "access plus 2 hours"
ExpiresByType application/x-javascript "access plus 2 hours"
ExpiresByType text/javascript "access plus 2 hours"
ExpiresByType text/x-javascript "access plus 2 hours"
ExpiresByType text/css "access plus 2 hours"
ExpiresByType image/gif "access plus 2 hours"
ExpiresByType image/jpg "access plus 2 hours"
ExpiresByType image/png "access plus 2 hours"
ExpiresByType image/x-icon "access plus 2 hours"
</IfModule>

# set compression
<IfModule mod_header.c>
<IfModule mod_deflate.c>
# Insert filter
SetOutputFilter DEFLATE

# Netscape 4.x has some problems...
BrowserMatch ^Mozilla/4 gzip-only-text/html

# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4\.0[678] no-gzip

# MSIE masquerades as Netscape, but it is fine
# BrowserMatch \bMSIE !no-gzip !gzip-only-text/html

# NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
# the above regex won't work. You can use the following
# workaround to get the desired effect:
BrowserMatch \bMSI[E] !no-gzip !gzip-only-text/html

# Don't compress images
SetEnvIfNoCase Request_URI \
\.(?:gif|jpe?g|png)$ no-gzip dont-vary

# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary
</IfModule>
</IfModule>