The URL used in the $next statement have to be encoded with URL encoding, like HATPCR play URLs. You can grab the link out of the CaveGen play URL, use "http%3A%2F%2Fwww.interguild.org%2Flevels%2Flevels%2F######.txt" where ###### is the level ID for Interguild levels, or run any URL through a URL encoder like this one: http://meyerweb.com/eric/tools/dencoder/