Thanks Slávek. Thought I would give this a try . It did resolve mirror.xcer.cz but the connection then timed out over 5 tries. Perhaps someone else might try this in case for some reason my IP address is a factor ? Ping mirror.xcer.cz also failed.
$ ping mirror.xcer.cz PING mirror.xcer.cz (91.241.23.210) 56(84) bytes of data. From 10.2.252.114 icmp_seq=2 Destination Host Unreachable
Here from Berlin, Germany. However, "10.2.252.114" looks suspicious to me, because netrange 10.0.0.0 - 10.255.255.255 is a private address usually used by internal networks of big company IIUC, i.e. my provider. When I ping the Freie Universität Berlin, I get this:
$ ping 130.133.4.195 PING 130.133.4.195 (130.133.4.195) 56(84) bytes of data. 64 bytes from 130.133.4.195: icmp_req=1 ttl=52 time=24.6 ms
I don't know if anything of this is relevant to your problem. At least I can confirm, that I'm not able to ping mirror.xcer.cz.
Hope this helps.
Regards, Stefan