Own plugin corrups plugin-activation

I am currently working on a simple plugin. Activating my plugin works fine. However, I can’t activate any other plugin as long as mine is active. Also, the edit links won’t work.

Any help with this would be great!
Below is the code for the main plugin file sponsoren.php

<?php  
/* 
Plugin Name: Sponsoren
Plugin URI: 
Version: 
Author: 
Description: 
*/  

// Enqueue Scripts
add_action( 'admin_enqueue_scripts', 'sponsoren_admin' );
add_action('admin_menu', 'sponsoren_custom_menu_page');
add_action( 'wp_enqueue_scripts', 'sponsoren_frontend' );

function sponsoren_custom_menu_page() {
   $page = add_menu_page( 'Sponsoren', 'Sponsoren', 'manage_options', 
       'sponsoren/sponsoren-admin.php', '', plugins_url( 'sponsoren/images/icon.png' ), 33 );
}

function sponsoren_admin() {
    wp_enqueue_style( 'sponsoren-admin-style', plugins_url('/stylesheet.css', __FILE__) );
    wp_enqueue_script( 'sponsoren-admin-script', plugins_url('/script.js', __FILE__) );
}

function sponsoren_frontend() {
    wp_enqueue_style( 'sponsoren-frontend-style', plugins_url('/sponsoren.css', __FILE__) );
    wp_enqueue_script( 'sponsoren-frontend-script', plugins_url('/sponsoren.js', __FILE__), array( 'jquery' ) );
}

// Create database table upon activation
function jal_install() {
   global $wpdb;

   $table_name = $wpdb->prefix . "sponsoren";

   $sql = "CREATE TABLE $table_name (
   id int NOT NULL AUTO_INCREMENT,
   name tinytext,
   bild tinytext,
   url tinytext,
   PRIMARY KEY id (id)
    );";

}

register_activation_hook( __FILE__, 'jal_install' );

// Ajax action used in sponsoren-admin.php
add_action('wp_ajax_sponsor_delete', 'sponsor_del');
function sponsor_del() {
    global $wpdb;
    $wpdb->delete( $wpdb->prefix.'sponsoren', array( 'id' => $_POST['deleteid'] ) );
    die();
}

// Widget
function widget_sponsoren_frontend($args=array(), $params=array()) {

    $title = get_option('widget_sponsoren_title');

    echo $before_widget;
    echo $before_title . $title . $after_title;
    // some more widget output

    echo $after_widget; 
}

wp_register_sidebar_widget('widget_sponsoren','Unsere Sponsoren', 'widget_sponsoren_frontend');

// Widget Options
wp_register_widget_control(
    'widget_sponsoren',     // id
    'widget_sponsoren',     // name
    'widget_sponsoren_control'  // callback function
);

function widget_sponsoren_control($args=array(), $params=array()) {

    if (isset($_POST['submitted'])) {
        update_option('widget_sponsoren_title', $_POST['widgettitle']);
    }

    $widgettitle = get_option('widget_sponsoren_title');
    ?>

    Widget Title:<br />
    <input type="text" class="widefat" name="widgettitle" value="<?php echo stripslashes($widgettitle); ?>" />

    <input type="hidden" name="submitted" value="0" />
    <?php
}