Can't update from official repositories [Error 409]

I can't update from official repositories with any pc from my local network. I think I haven't changed anything from my router but I also got an error report sentece telling me official repository isn't signed:

E: Failed to fetch http://es.archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease  409  Conflict [IP: 104.19.139.75 80]
E: The repository 'http://es.archive.ubuntu.com/ubuntu bionic-updates InRelease' is not signed.

I'm using Ubuntu 18.04.3 LTS

What can I do? I haven't found anything useful on the Internet.

Solution 1:

The easiest way is to go directly to Software & Updates → Ubuntu Software. In that tab, where it says "download from" change the one you have (you will have the one from your country) and change it to Main Server, so you don't have to edit the sources.list file. After few days if you want to use the one in your country again (see if it works again) and if it does not continue with the main server if it does not give you problems.