If you say you can connect to other pools just fine then it means that there is a problem with the pool you are trying to connect to.
I'd suggest you try check the exact pool address and port number from nicehash, could be a typo.
Or, another possible problem is that the actual pool server is down/offline. In this case you could always try connecting to nicehash's US instead of EU server.
I tried the US server, and i have the same problem there.
I guess i will have to wait and try again tomorrow if they have some kind of issues going on right now.
I am pretty sure everything is ok at my end.