The URL works fine from a browser and cURL.
I do see that it's blocked when using WGet because of your User Agents to Block configuration in the .htaccess Maker. Maybe this is the problem (even though I personally never had that with Google — they use a different a user agent as far as I can see on my sites). Anyway, let's give it a shot.
Go to your site's administrator, Components, Admin Tools, .htaccess Maker
Find the “User Agents to Block, one per line” area.
Find the line that reads
WGet
and remove it.
Click on Save & Create .htaccess.
If you still have a problem please try disabling the “Block access from specific user agents” option and Save & Create .htaccess again.
If this still doesn't help your problem is unrelated to Admin Tools. Maybe you are using a third party extension, host feature, CDN or similar to block entire countries? This could very reasonably cause the sitemap to be unreadable by Google's server with an IP address which belongs to a US-based company.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!