I'm facing the same issue here. App is working fine, servers are OK, the credentials I provided to Apple are OK (double checked it), I have hundreds of users using this app normally. The only place where this login fails is on AppStore review. And they only attach a screenshot of the message (Could not login, connection problems), but they never provide any additional information about their environment and network settings. I also tested with a Nat64 WIFI network following their instructions on the documentation, and it worked fine here. I don't have any idea of what is causing this issue on AppStore.

I'm facing the same issue here with AppStore Connect. "We discovered one or more bugs in your app. Specifically, the app loaded indefinitely upon login attempt." All of my tests work. On Apple side, kind of 25% passes and 75% is rejected. It seems to be random, but it's getting worse in the last days.