If all pages always show a 500 internal server error:

• Make sure you didn't upload to a /cgi-bin/ directory. WSN Links is PHP, not Perl, so it doesn't belong there.
• A problem in the .htaccess file can cause a 500 error. Check if the error goes away when deleting .htaccess. If so, work through it to find the line that's killing it (or ask for support).
• A problem with PHP itself can cause a 500 error. Check if the servertest.php file loads, if not then the server's PHP install is likely messed up and this needs to be addressed by the web host.
• A bad chmod value can cause a 500 error in some server configurations. Servers that don't allow 777 directories sometimes give these errors when using 777 -- try setting the directory to 755 instead (and be sure to change the Settings->System Settings value to 775 as well if that's the issue). Files may need to be 644. Test with the servertest.php file to see if you can find permissions that allow that to load.
• There may be a script error which is being hidden from view, despite WSN's attempts to be allowed to handle the error display decisions (sometimes the error happens before WSN has taken control). In that case, you need to uncover what the real error is. If there's an error log txt file in the directory where you installed WSN, open that in a text editor and scroll to the end to see what the most recent error message was. If not, you may need to adjust the php.ini or the PHP configuration options your web host offers you to tell it to display errors.

If all pages temporarily show a 500 internal server error:

• Your web host was probably experiencing difficulties for a bit.

If pages sometimes show a 500 internal server error:

• A few servers require a -f parameter in sending emails, though the vast majority should not use -f. Go to Admin Panel -> E-Mails -> E-Mail Preferences and set "Use -f parameter in sending mail?" to "yes". See if that clears up the error. If it doesn't, turn the -f off again.
• Other sorts of malformed mail may also cause a 500, such as a bad "to" address, though WSN automatically avoids sending emails to a blank recipient.
• A problem in the .htaccess file can cause a 500 error. Check if the error goes away when deleting .htaccess. If so, work through it to find the line that's killing it (or ask for support). As a temporary measure until solving the .htaccess problem, disable URL rewriting at Admin -> Settings -> SEO so that your site will be navigable.
• A bad chmod value can also cause a 500 error. Servers that don't allow 777 directories sometimes give these errors when using 777 -- try setting the directory to 755 instead, and the files in the base and admin directories to 644.
• If you're running PHP as CGI then 500 errors become much more common and can mean many more things. See http://encodable.com/internal_server_error/
• There may be a script error which is being hidden from view, despite WSN's attempts to be allowed to handle the error display decisions (sometimes the error happens before WSN has taken control). In that case, you need to uncover what the real error is. If there's an error log txt file in the directory where you installed WSN, open that in a text editor and scroll to the end to see what the most recent error message was. If not, you may need to adjust the php.ini or the PHP configuration options your web host offers you to tell it to display errors.