The error you have is usually received when something is stopping you from connecting... (duh).
It seems that you might be running bitcoind with -connect=0, or something similar ( bitcoin core not having access to any network, as above user mentioned.)? Could you check that?
I'm always use the same bitcoin.conf - and only one server refuse to work with testnet
You might have a problem with connection to other peers.
Have you changed the bitcoin.conf in the testnet subdirectory?
What does your client return when you run "bitcoin-cli -testnet getpeerinfo" ?
I just run bitcoind in testnet, and now its connected to 6 nodes. I dont know why few days ago I had a problem, but thank you anyway