Swapping ftp.udc.es for ftp.cixug.es
While both ftp.udc.es and ftp.cixug.es share the same IP, the
certificate is only assigned to the ftp.cixug.es domain (to *.cixug.es,
in fact). This causes the line
deb https://ftp.udc.es/debian stable main
to fail with a certification error.
For this reason the mirror list should have an entry for the cixug
server, in place of the udc one.