After updating to WordPress 5.8, you may have serious problems with your sidebars, because the new version of WordPress manages the widgets with the blocks and not anymore with the classic widgets.
If you have this kind of problem, add this code to the functions.php of your child theme or in a functional plugin:
add_filter( 'gutenberg_use_widgets_block_editor', '__return_false' ); add_filter( 'use_widgets_block_editor', '__return_false' );
Then, you may need to rebuild your sidebar.