Due to the setup of Classic compared to Lightning, the conversion app can sometimes render incorrectly when switching to Lightning. Below are some instructions are how to correct this.
1. When in Lightning, switch back to Classic.
2. When back in Classic, go to page setup. Then drag the conversion app over to the sidebar.
3. Once the app has been moved, save your changes and switch back to Lightning. The app should now be appearing and rendering correctly.
If you run into any other issues, please reach out to the support team, firstname.lastname@example.org