Funny enough, everytime I would save the .htaccess file and reload your admin page, I can save it. After it doesn’t work. For a reason my HT access is replaced. So I could save some settings in reloading the page.
Now that I finally can have access to the user dashboard page, I see that there is an error displayed on top of the page which is:
Warning: array_merge(): Argument #2 is not an array in /home/…/wp-content/plugins/download-manager/libs/class.UserDashboard.php on line 35
Please advice and perhaps upgrade your plugin, the library you are using is maybe conflicting. (tried with default theme and no plugin).