Thanks for posting!
We might be able to resolve this by redeploying the web publishing engine. To do this, open the admin console and head to the FileMaker Server Overview. On the right side of this screen, click Edit Server Deployment and follow the prompts from there.
This could just be an omission in your first post, but you do have FileMaker Server Advanced, not FileMaker Server installed?
FileMaker Server does not support Instant Web Publishing.