Fatal error: Cannot declare class Download_Attachments_Shortcodes, because the name is already in use in /home/mjlp/ftp/www/zzzzzzz_marek/szkolaaleksander.pl/wp-content/plugins/download-attachments/includes/shortcodes.php on line 11
This points to an issue with the same PHP class declared twice.
In this case it would happen if you have for example Download Attachments installed twice, which is hardly possible.
Maybe you try to declare the Download_Attachments_Shortcodes in your theme files?
Can you do some checkings in your files?