When not logged in, the first time the Frontend Submission form is submitted it fails. It simply reloads the page without error and clears all fields. When re-entering content and submitting it again, it works fine. I disabled all plugins, switched to the TwentyTwenty theme and reduced the code to the shortcode to the bare minimum. It still did not work. (Switched it back now.)
Sample Shortcode: [mb_frontend_form post_fields="title,content" post_type="post"]
More complex page I am trying to make work: https://mm.thewebtailors.net/submit-a-museum/
Running all the newest available versions.
Any help is appreciated. I can set up a staging site for you to work with.