I feel like my problem must be related to inability for Discourse to access the image upload cache either when creating a post with images or editing an existing post with images. I am also completely upgraded to 2.9.0.beta4. In beta1 I did not see this problem. I am thinking it occurred in either beta3 or beta4. Whenever editing an existing post with images, I get a 500 error when attempting to save the edit. When creating a new post with images, I get:
If I either edit or create a post with text and hyperlinks without embedded graphics, it works fine.