By debugging the APK this is what I catch from the Console:
[Construct 3] AJAX request to 'https://xxxxx.playfabapi.com/Client/LoginWithCustomID' (tag 'Login') failed:
ProgressEvent
isTrusted: true
lengthComputable: false
loaded: 0
total: 0
type: "error"
target: XMLHttpRequest {readyState: 4, timeout: 0, withCredentials: false, upload: XMLHttpRequestUpload, onreadystatechange: ƒ, …}
currentTarget: XMLHttpRequest {readyState: 4, timeout: 0, withCredentials: false, upload: XMLHttpRequestUpload, onreadystatechange: ƒ, …}
eventPhase: 0
bubbles: false
cancelable: false
defaultPrevented: false
composed: false
timeStamp: 5670.0999999993655
srcElement: XMLHttpRequest {readyState: 4, timeout: 0, withCredentials: false, upload: XMLHttpRequestUpload, onreadystatechange: ƒ, …}returnValue: truecancelBubble: falsepath: []__proto__: ProgressEvent
_TriggerError scripts/c3runtime.js:574
xxxxx.playfabapi.com/Client/LoginWithCustomID:1 Failed to load resource: net::ERR_CACHE_MISS