Slack has decided to remove the XMPP and IRC gateways, they will cease functioning on May 15th. It's a real shame, because these were the only usable interfaces to slack, both the web UI and the desktop app hog memory like a separate browser session, but there are still some solutions you can use to avoid using them.
I set this up 2 years ago, but today I had to add another Slack network to add, and it took me an hour to figure out. The connection to ZNC was working fine, but when ZNC was trying to connect to the Slack server, I always got "Invalid user name or password" from slackbot. Slack doesn't explicitly tell you, but the username it gives you is supposed to be your nickname, and your ident, and you have to set this up before making a connection attempt.