Comment 7 for bug 1329202

libshout adds the prefix "/" if there is none:

So it actually does not matter if the mount point in Mixxx has a / or not.

the mounpoint is used unescaped inside libshout:
  rv = sock_write(socket, "GET /admin/metadata?mode=updinfo&mount=%s&%s HTTP/1.0\r\nUser-Agent: %s\r\n%s\r\n",
    self->mount, encvalue, shout_get_agent(self), auth ? auth : "");

This mean that it must at least not contain blanks.

By the way:
Shoutcast (SHOUT_PROTOCOL_ICY) does not support mountpoints.
So we may gray-out the field if Shoutcast is selected.