If you see 500 error on your website (internal server error), you should check file and folder permissions via FTP. Incorrect permissions:

rw-rw-rw- (666) — Everyone can read and write to the file.

rwxr-xr-x (755) — The owner has read, write, and execute permissions; the group and others have only read and execute.

All other files, php scripts etc must be set to 644:

rw-r--r-- (644) — Only the owner has read and write permissions; the group and others have read only.

Except .cgi files (755 required for perl and cgi scripts). Please note that cgi files will work in /cgi-bin directory.