What this basically means is that is cannot resolve the address of the server you've given it. Check you've got a network connection, whether or not you can ping that address (you're not so interested in whether it can ping, but whether it can resolve - which will tell you if its a problem generally with that address or just that program doing something funky), whether or not you can resolve other names (you might want to try going to a website you've never been to before), etc.
Sounds like you are with BT, do you ever disconnect your modem when you go to bed? Or Do you constantly keep it running 24/7, if you do, i suggest rebooting your ROUTER, if that doesn't help, reboot your modem too.
It happens with my router too, i have to just pull the AC/DC adapter out and plug it back in again and it works. Sometimes the modem is the problem. So same deal with that. Let me know if it works for you!