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As you can see for yourself it returns a 303 See other HTTP header as it should. It doesn't return a 403. Apparently you have enabled one of the other .htaccess protections which only get triggered when your site is accessed by the third party service. My GUESS is the "Block access from specific user agents" option.

Patrick, the 303 is how Joomla! issues redirections on multilingual sites. It doesn't come from the .htaccess file. I'm not going to apologize for Joomla! core code that I've not written! In any case, I don't see a redirection issue coming from code I've written and which I can help you with. In fact, there is no redirection issue.

FYI all search engines treat 301, 302 and 303 in pretty much the same way: they assume you've screwed up and check if the redirection stays the same over time. What the SEO guys write on blog posts is complete bullshit. When we moved our site from Joomla! 1.5 to 2.5 and from 2.5 to 3.2 our URLs changed. We issued redirections from the old to the new URLs. I used 302 and 303 HTTP responses each time. Our search engine ranks didn't take a hit and our updated links appeared in search indexes within two or three days. This is when I started not believing in SEO guys any more than I believe in the existence of the Flying Spaghetti Monster.

I did enabled the “Block access from specific user agents — Do you mean this have disabled that third party website canonical test to access the site?

Yes, that's what I think. Most likely they're using curl or wget to get the responses from your site. Since these user agents are in the default list you're blocking them.

I was thinking this was really domain specific?

Please read the documentation. This feature blocks request based on the User Agent identification string, commonly called "UA string", "User Agent" or "UA". This only depends on the remote software used to access your site. It has absolutely nothing to do with the domain name whatsoever.