There is no need to post the same request for help multiple times, in this case 4 times. When someone on this volunteer forum have an answer they will post a reply. Patience is required on your part.
Perhaps the site your system is being redirected to is having some problems or it is in the middle of an update.

Ken Schneider 

On Jun 14, 2021, at 12:04 PM, Marco Calistri <PY1ZRJ@outlook.com> wrote:

 Since some days I get errors like this (sorry to be in my language but I'm sure you understand the issue).

When it happens I delete the specific repository or disable then execute the zypper ref again.

- Errore di download (curl) per 'https://download.opensuse.org/repositories/Education/openSUSE_Tumbleweed/repodata/repomd.xml':
Codice di errore: Curl error 16
Messaggio di errore: 
 - Can't provide /repodata/repomd.xml
Is it a known problem at opensuse?

Thanks for your attention.

Regards,
-- 
Marco Calistri
Build: openSUSE Tumbleweed 20210606 
Kernel:5.12.9-1-default
Desktop: XFCE (4.16.0)