Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 79 bytes) in /home/xxx/public_html/xxx/wp-includes/wp-db.php on line 1542. This error message is due to the activating many large, high-functionality plugins in the single WordPress installation. You can fix this problem easily by using the steps given below:
Step 1. Login your server using the FTP.
Step 2. Download wp-config.php located in the root of your WordPress installation.
Step 3. Open the wp-config.php file in the editor of your choice.
Step 4. Add this line after the <?php tag define(‘WP_MEMORY_LIMIT’, ’64M’);
Step 5. Upload and replace wp-config.php file.
Once this has been done, you should have no problems activating large and many numbers of the plugins. I hope it will help to someone.