WARNING: The “PHP Max Input Vars” parameter is set to 1000 which is not enough for our Theme Settings. You need to increase it to 2000 or higher in your PHP configuration. Contact your hosting provider for help on this matter. Otherwise, you will not be able to save Theme Settings.
After deactivation the Elementor Pro, the Thank you page started to appear well – https://take.ms/SIbs8 so the issue is that you have created a custom Thank you page via Elementor Pro that is working wrong. Remove that custom thank you page and use the default one.
It seems that somehow the Elementor Pro affects that page even after removing the custom Thank you page. If you disable it, it will work well. So I suggest you contact with the Elementor support so they can debug the issue.