I am having the same problem. I have tried WordPress v. 2.7.1 and v. 2.6.5. I have tried erasing my entire site and reloading WordPress. I've tried reloading it with Fantastico and with SimpleScripts. I've tried putting GoCodes in a subfolder in the Plugins folder and I've tried putting GoCodes directly in the Plugins folder without a subfolder. I've tried changing the .htaccess file and I've mostly left it as-is.
I'm using PHP v. 4.4.9 on HostMonster.
Since this other guy had this problem when he upgraded, I'll try installing a previous version of GoCodes instead and see if that fixes it.