Problem with connecting to MySQL/MongoDB/Postgres etc. database "Could not load file or assembly.."

I am trying to connect to a MySQL database. Unfortunately I always get errors like this:

System.IO.FileNotFoundException: Could not load file or assembly 'System.Diagnostics.DiagnosticSource, Version=5.0.0.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. Das System kann die angegebene Datei nicht finden.
File name: 'System.Diagnostics.DiagnosticSource, Version=5.0.0.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'
   at MySqlConnector.MySqlConnection.OpenAsync(Nullable`1 ioBehavior, CancellationToken cancellationToken)
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start[TStateMachine](TStateMachine& stateMachine)
   at MySqlConnector.MySqlConnection.OpenAsync(Nullable`1 ioBehavior, CancellationToken cancellationToken)
   at MySqlConnector.MySqlConnection.Open() in /_/src/MySqlConnector/MySqlConnection.cs:line 373
   at AdminTools.AdminToolsDatabase.Connect() in C:\Users\Milan\source\repos\AdminTools_Server\AdminTools_Server\AdminToolsDatabase.cs:line 30

"Das System kann die angegebene Datei nicht finden." should mean something like "The system cannot find the specified file."

using System;
using MySqlConnector;

namespace AdminTools
{
    class AdminToolsDatabase
    {
        private static string connectStr = @"server=127.0.0.1;uid=Adminii;pwd=123;database=server_db";
        private static MySqlConnection cnn = null;
        public AdminToolsDatabase()
        {
            cnn = new MySqlConnection(connectStr);
            Connect();
            Console.WriteLine("Connection established");
        }

        private bool Connect()
        {

            if(cnn != null || cnn.State != System.Data.ConnectionState.Open)
            {                
                try
                {
                    cnn.Open();
                    Console.WriteLine("connected.");                    
                }
                catch (Exception ex)
                {
                    Console.WriteLine("Error connect mysql");
                    Console.WriteLine(ex);
                }
            }
            return false;
        }
    }
}

Oh I get this error when I run the programm which uses my dll. If I use invalid props for the mysql connector string it will give me an error for that. But as soon as I call cnn.Open(); it crashes.

I have tried using newer and older versions of the mysql connector - without any success. The Mysql.Data packet did also not work. I also tried to use postgres and mongodb instead. But I always get the same kind of errors (just with different dll names). Any ideas?

(I also tried switching between 4.6, 4.7 and 4.8 without any success)

I would appreciate any help.. cause I really don't want to use MS SQL Server 2019..