No updates for Trisquel 8

I check manually for updates every day, and since yesterday March the 7th, 2021, the Software Update from System Administration is failing. The error message is just a succint "The download of the repository information failed. Check your Internet connection". My Internet connection is working OK for everything else, so that doesn't seem to be the problem.

When the Software Updates shows the dialog box with the error, the first time I click on "Try again" only to the error to be repeated, the second time I try to circunvent the error by clicking on "Settings", then on the dialog box "Software and updates" selecting the "Trisquel software" tab and switching the repository to another, and then after switching I am prompted for reloading the package information, which I accept, and then I get the error again but this time with a detailed text:

W:The repository 'https://archive.trisquel.info/trisquel flidas-security Release' does not have a Release file., W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use., W:See apt-secure(8) manpage for repository creation and user configuration details., E:Failed to fetch https://archive.trisquel.info/trisquel/dists/flidas-security/main/binary-i386/Packages server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none, E:Some index files failed to download. They have been ignored, or old ones used instead.