BUTT, Mixxx or RadioDJ refusing to connect is nearly always one of five things, and the error messages are unhelpfully similar — most of these look like a wrong password.

1. The Type Is Set to Shoutcast

It has to be Icecast 2. They are different protocols, and a Shoutcast client fails against your station with an authentication-shaped error even when every value you typed is correct. This is the most common cause by a distance.

2. There Is a http:// in the Server Field

The server field wants a hostname and nothing else — no http://, no https://, no trailing slash, no path. Mixxx is especially strict about this, and pasting a URL into it is the second most common cause.

3. The Station Is Off Air

Unlike the browser studio, an encoder does not switch your station on. Press Start AutoDJ on the dashboard first, then connect.

There is a related trap: a station that is on air with nothing attached and nothing playing takes itself off air after ten minutes. If you switched it on, went to configure your encoder, and took a while, it may have stopped underneath you. Switch it back on and connect promptly.

4. The Mount Has the Wrong Number of Slashes

Use the Mount exactly as shown on the Go live page, with the leading slash (/my-station). BUTT also accepts it without the slash and adds it itself; a mount with two slashes, or a typo in the name, is refused. Copy it with its Copy button rather than typing it.

5. Something Else Is Already Connected

A station takes one broadcaster at a time. If a browser studio is live, or an encoder is still connected from earlier, a second connection is refused. Check what the station page says it is live from — it names the software where it can — and disconnect that first.

An encoder that crashed rather than disconnecting cleanly can hold the slot for a short while. Taking the station off air and back on clears it.

Other Things Worth Ruling Out

The username is source, not your email address and not your station name.
Your plan. Encoder broadcasting needs Pro. On Free the credentials are not offered at all — if Go live has no Use DJ software instead, that is why. See Free and Pro.
A rotated key. If you pressed New key, every encoder holding the old one stops working until you paste the new one in.
A restrictive network. Some office and campus networks block outbound streaming ports. Try tethering to a phone as a test — if it connects there, it is the network, not the setup.

Still stuck? Email hello@gocast.fm with the software, its version, and the exact error text. See also broadcasting from BUTT, Mixxx or RadioDJ.