The server is having a buggy issue with MC 1.7.4 and some servers. It seems to affect some but not others.. It's mentioned in some comments further back as well, and Mojang's ticket system currently has it as unresolved.
The short of it is, the server occasionally crashes, and that's fine, it comes back up and people can play.
But there are also 'crashes' that boot everyone, but the server still shows that it's up and working and online, and that it has people logged into it, even though they're not. When this is occurring and someone tries to join it, it'll mention and io nethandler error. The only way to get the server back in working order when that starts happening is someone human needs to manually restart it to make it available again.
So just keep that in mind when you try to join.