Error 404 on things after "/"

Thanks so far!
What I mean is, that no CSS-File, even all other files, are not loading. Sorry for my english :-)
baseURL has a trailing slash. I try this three ways to check what is going on:

Code:

<link rel="stylesheet" href="<?= base_url('dashboard/template/xyz.css');?>">

<link rel="stylesheet" href="dashboard/template/xyz.css">

<link rel="stylesheet" href="/dashboard/template/xyz.css">

I check the network traffic (F12 > Network traffic) and see only 404 errors on all loaded files. If I go to "Answer", I can see the error, which I will call the "Oops"-Error :-) Happens sometimes :-)
The answer looks as every file, even in public folder, are loaded like a controller, Because "Dashboard" and "Template" in "Message" (line 4) are the first in directories in /public like in my HTML-markup above.

PHP Code:

{
 "title": "CodeIgniter\\Exceptions\\PageNotFoundException",
 "type": "CodeIgniter\\Exceptions\\PageNotFoundException",
 "code": 404,
 "message": "Controller or its method is not found: \\App\\Controllers\\Dashboard::template",
 "file": "/var/www/vhosts/[...]/vendor/codeigniter4/framework/system/CodeIgniter.php",
 "line": 992,
 "trace": [
 {
 "file": "/var/www/vhosts/[...]/vendor/codeigniter4/framework/system/CodeIgniter.php",
 "line": 992,
 "function": "forPageNotFound",
 "class": "CodeIgniter\\Exceptions\\PageNotFoundException",
 "type": "::",
 "args": [
 "Controller or its method is not found: \\App\\Controllers\\Dashboard::template"
 ]
 },
 {
 "file": "/var/www/vhosts/[...]/vendor/codeigniter4/framework/system/CodeIgniter.php",
 "line": 353,
 "function": "display404errors",
 "class": "CodeIgniter\\CodeIgniter",
 "type": "->",
 "args": [
 {}
 ]
 },
 {
 "file": "/var/www/vhosts/[...]/index.php",
 "line": 44,
 "function": "run",
 "class": "CodeIgniter\\CodeIgniter",
 "type": "->",
 "args": []
 }
 ]
}