At any time while creating the new forum in the control panel did you use your Back button in your browser? That might have caused this.

If you FTP to your server, do you see a Forum directory in your nonCGI directory for the problem forum? If so, does it have permissions set to 777?

If the answer is no to the first part, try creating a ForumX directory with permissions set to 777.

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Doug
darksaber.hispeed.com
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.