I installed the RSS reader FreshRSS on my mutualized OVH hosting, and it turns that some websites (also hosted on OVH, ah ah) protects themselves with HTML captcha pages (instead of the RSS XML). And since FreshRSS is completely a robot and will never solve captcha, it just gives an unclear error. Github ticket.

In the captcha page:

• [domain name] is protected by Imunify360
• We have noticed an unusual activity from your IP [REDACTED] and blocked access to this website.