If you are ABSOLUTELY sure that you've done everything as suggested (chmodded pages to 777, etc.), you might have mod_security enabled on your server. Please try to place only “Hello world” (without quotes, and it is important that no HTML tags are placed in text) and if you can save that, but not page content, than you can be sure that it's mod_security what's causing problems.

If your server is installed as server module, and not as CGI module you could try to create blank text file, name it .htaccess and place it in your manage/ folder, and place following lines in it. If such file already exists, just append following code at the end of it:

SecFilterEngine off

If this does not help, you have to contact your website hosting provider tech support and ask them if mod_security is activated for your domain? If yes, ask them to disable it.