Yes correct. Issue is in the server itself. That is the reason both default RSS widget and SRR is failing.
Anyways, what is the other RSS URl which is successful ?
Did you try the URL with http or https ?
Is any other part of WordPress affected with this same issue ? Example, wordpress updates page, plugins search page, theme search page etc.. ?
You might have to talk to the host and check if they did change any config.
This is because if Widget was working fine all this time and suddenly it breaks, then some problem with the host or the RSS URL.
Thanks,
Aakash