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)