Either:

• delete the .htaccess file in the Simple Invoices folder
• or enable mod_rewrite in Apache

refer: http://simpleinvoices.org/forum/discussion/1379/error-500-on-first-install/#Item_3

Before you start changing your server configuration as suggested above, try putting ”” around the password in the config.ini file.

Perhaps the developers will add the quotes in the default config.ini file to save others from this easily avoided hiccup.