Hi, Brandon
Did you delete cached available_app_components.json file before performing this steps?
If no, please remove the <IDE_SYSTEM_DIR>/jmix-studio folder and try with NIGHTLY build again.
As we can see in the files you attached above, Jmix plugin saved invalid available_app_components.json file to the cache at some moment. It is actually “site is on the maintenance” response. So deleting this file should resolve the issue. But in your case this invalid file saved again for some reason. That is why I asked about proxy, as our site is available now, and doesn’t return such response.
New NIGHTLY plugin build has fix for that case, it would save only valid JSON file to the cache.