http error on uploading images

Hi, suddenly this error appeared today and I can’t solve it.

Like every sunday, I did a complete backup via ftp of my site, after posting an article. Now I’m trying to post another article and when I try to upload an image this errore comes out.
I did nothing on the settings of my blog, no plugin updates, nothing.

I read something about editing the .htaccess file, but i don’t have it on the root of my site.

How can I solve it?