Problem with protected RSS url

Hello!

Since 9th of September (that I know) RSS protected url is not working properly.

I can’t access to the URL, despite I login with a user with the adecuate capabilities.

I have tried to disable all plugins and the problem is still there.

My hosting provider is Siteground.

Do you know what could cause this error?

Thank you!
Regards!

The page I need help with: [log in to see the link]