Update: It's now working in iOS on device (no clear understanding why it failed before – we didn't change a thing). It still fails in Mac Catalyst with the error Application with identifier <...> is not associated with domain <...> Disappointed by Apple Technical Support here: no response, no help, just silence. :-/

Update 2: After updating the System to macOS 15.0.1, it works in Mac Catalyst too. No changes in code or configuration. I'm worried it might break in production without us knowing why. Not great.

Here's our configuration. webcredentials:ourdomain.tld?mode=developer https://ourdomain.tld/.well-known/apple-app-site-association { "webcredentials": { "apps": [ "XXXXXXXXXX.our-app-bundle-id" ] } } Things we double-checked: Tried the Xcode configration with and without ?mode=developer The SSL certificate on our server is valid There are no HTTP redirects The reponse's Content-Type is application/json Apple's CDN returns the correct JSON file The JSON is valid 'sudo swcutil verify -d ourdomain.tld' returns the correct entry for the App ID There is no typo in the App ID, it's identical to to what the system is mentioning in the error message.