We’re aware of a loading issue with the add-ons page on the website. The add-ons page in the app is loading for me, though.
If you consistently see the 503 error in the app, please check that you’re not blocking requests to Local’s add-on server domain at https://api.localbyflywheel.com/ via any security or antivirus software at the system level. You may need to whitelist that domain, along with the other domains listed here.