This is Rae from Shopify. Thank you for posting this question, and welcome to the Community.
I'm glad to lend a hand to help ensure the sections on your theme start loading correctly within the Theme Editor.
As a starting point, can you please confirm which theme you're using when this issue occurs? Did you download this theme from the Shopify Theme Store, or from a third party developer?
Based on the screenshot you included, it looks like you're encountering this issue on a Chrome browser. Does the same thing happen on other browsers, like Safari or Firefox? If not, then I recommend clearing your Chrome cache and trying to view the theme once again.
Have you also noticed this happen with other themes in your Shopify admin? To test this, login to your admin, click on 'Online store > Themes' and scroll down the page to the 'Theme library'. If you have any themes added here, please click 'Customize' beside one of these themes and let me know if the sections load correctly.
I also recommend testing this on a brand new theme. To do this, just download a free copy of the newest version of the Debut theme via this link. Once this theme is added into your admin, click 'Customize' beside the theme and check to see if the sections load within the Theme Editor.
Please keep me posted on how this troubleshooting goes.
I look forward to hearing back from you!
Thank you for reaching out about this issue on your Venue theme.
I appreciate you completing the troubleshooting steps I shared above and testing this out on various browsers, and on different theme copies. Since the issue occurs on multiple browsers, and only happens on one theme, it sounds like these problems may be tied to the Venue theme specifically.
Venue is a third-party theme that's developed and supported by Safe As Milk, who are external to Shopify, so I recommend reaching out to them directly for more information on how to resolve this issue with the theme. You can contact these developers by following the steps outlined here, or by visiting this link and clicking the 'Support' button on the righthand side of the page.
Once you contact Safe As Milk about this, they should be able to help look into this further on their end.
Best of luck!