In case it helps in troublshooting, if I remove this line from the .htaccess file:
AddHandler application/x-httpd-ea-php83___lsphp .php .php8 .phtml
The Download Manager works fine. But this line is generated automatically by cPanel so it gets added back, see below how it looks in the .htaccess:
# php — BEGIN cPanel-generated handler, do not edit
# Set the “ea-php83” package as the default “PHP” programming language.
<IfModule mime_module>
AddHandler application/x-httpd-ea-php83___lsphp .php .php8 .phtml
</IfModule>
# php — END cPanel-generated handler, do not edit