Error when update something

Hello i have a problem with this site: arilosa.it When i add a new post or product it give me this error and depends form WPFastestCache. I can’t deactivate the plugin. The problem started yesterday.

Here the error: Un errore di?E_ERROR?è stato causato nella linea?63?del file?/var/www/vhosts/arilosa.it/httpdocs/wp-content/plugins/wp-fastest-cache/inc/varnish.php. Messaggio di errore:?Uncaught Error: Call to a member function get_error_messages() on array in /var/www/vhosts/arilosa.it/httpdocs/wp-content/plugins/wp-fastest-cache/inc/varnish.php:63 Stack trace: #0 /var/www/vhosts/arilosa.it/httpdocs/wp-content/plugins/wp-fastest-cache/wpFastestCache.php(1788): VarnishWPFC::purge_cache(Array) #1 /var/www/vhosts/arilosa.it/httpdocs/wp-content/plugins/wp-fastest-cache/wpFastestCache.php(78): WpFastestCache->deleteCache() #2 /var/www/vhosts/arilosa.it/httpdocs/wp-content/plugins/wp-fastest-cache/uninstall.php(18): wpfastestcache_deactivate() #3 /var/www/vhosts/arilosa.it/httpdocs/wp-admin/includes/plugin.php(1316): include_once('/var/www/vhosts...') #4 /var/www/vhosts/arilosa.it/httpdocs/wp-admin/includes/plugin.php(972): uninstall_plugin('wp-fastest-cach...') #5 /var/www/vhosts/arilosa.it/httpdocs/wp-admin/includes/ajax-actions.php(4766): delete_plugins(Array) #6 /var/www/vhosts/arilosa.it/httpdocs/wp-includes/class-wp-hook.php(324): wp_ajax_delete_plugin('') #7 /var/www/vhosts/arilosa.it/httpdocs/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters('', Array) #8 /var/www/vhosts/arilosa.it/httpdocs/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #9 /var/www/vhosts/arilosa.it/httpdocs/wp-admin/admin-ajax.php(192): do_action('wp_ajax_delete-...') #10 {main} thrown

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