Well, this is a local server and it's an HTTP 500 server error. Why not look at the Apache error log and read exactly why it throws the error? When we have an error log, it's counter-productive having me guess :D
About the disappearing .htaccess. On Linux, MacOS X and all UNIX derivatives dotfiles (files whose name starts with a dot) are hidden. Since you are running your site on a NAS, my best guess is that it's using Linux and that's why you are NOT supposed to see .htaccess, even though it's there.
Though, one quick pointer: most local servers do not load mod_rewrite by default. It's the usual suspect for white pages on a local server (Joomla! uses mod_rewrite to serve SEO URLs; its .htaccess loads it even if you have disabled SEO URLs in Global Configuration). Make sure that it's being loaded, by editing your Apache configuration file. If, however, this is a PHP error... guess what? You have to read the server's error log.
Nicholas K. Dionysopoulos
Lead Developer and Director
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