I went to your website and saw that I still have an account on it. I checked the admin credentials (api_key and publishing_username) and they are valid. I didn’t try creating a post.
One thing that can make it impossible to publish a post from WordPress to Discourse is if you publish a post from WordPress to Discourse, then delete the post on Discourse and then try to republish the same post from WordPress do Discourse. Possibly this is what’s causing the problem you’re having. It’s definitely something that needs to be fixed. To do this, an ‘unlink-post’ setting needs to be added to the Publish to Discourse meta box. I’ll fix that right away.