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You may try to change your php version to 5.3, you can do it in AlterVista control panel (Resources -> Edit php settings)

Yuu had a broken .htaccess in your /administrator/ folder, commenting out part of it did the trick.

umsfp:

I can see your website, is the error still there? If so can you please post some example url?

Remove # from

# RewriteBase /

And let us know

Please paste here the content of your .htaccess file

Yes, it is necessary

Have you tried the password protection function in your contol panel (file management)?