Firebase Functions cannot find address associated with hostname?

I use Firebase Functions with my Android app, and for the past few days, some users have reported an "Internal" error when they make an app request. I have added some logs right at the start of the server-side function and noticed that the logs were never reached. The logs were right at the start of the function, so if there was any kind of crash, at least the log was shown. So I asked one of the users to help me debug this and test one version of my app with a popup of the error in the app and it seems that the function is not finding my server address? I have been working with this user for the past few days and we have tried:

• Clearing the app cache;
• Reinstalling the app;
• Rebooting the device;
• Turning off and on the device WiFi;
• Rebooting the WiFi router;
• I have updated the google-services.json;
• I have added FirebaseApp.initialize(context) at the Application class;

None of that worked. This just happens with Firebase Functions. Firebase Firestore is still working in the app. Does anyone have any clue about what is going on? Note that the user was able to call the functions without problems, but from a moment to another this issue started happening. I suspect that the user device is using IPv6 and that may be causing the problem.

EDIT: Just for the sake of completeness all my functions are implemented like this:

Android app:

val data = //some String
Firebase.functions.getHttpsCallable("myfunctionname").call(data).addOnCompleteListener {
    //the result
}

Javascript function:

exports.myfunctionname = functions.https.onCall((data, context) => {
  //do something
  return //something
})

from Firebase Functions cannot find address associated with hostname?