We've had trouble reproducing this issue on our end. After some testing with a small app on MacOS and iOS, we have not been able to see the error you are encountering using Unity 2021.3.8.
If you could, please check out the app here and let us know if this issue is still persisting. It is a barebones authentication app and it should run on multiple devices. The Authentication.cs script has all the gory details. Just open up the Main scene under Assets > App > Misc > Main.unity to get it running. You will also need to add your brainCloud credentials, but you can copy the Authentication example template in brainCloud to test the app.
Please let us know how this app works on your end and if any issues are encountered! You should be able to click on any error logs in the in-app console to copy the log to your device's clipboard.
As for your specific issue, my best guess is that perhaps an external library of sorts might not be supported with brainCloud, such as the Newtonsoft Json.NET library. For JSON de/serializing, brainCloud comes with JsonFX. You can also make use of Unity's own JsonUtility for structured JSON de/serialization. Both the app I've shared and the examples on our GitHub make use of JsonFX extensively.
If you'd like to know more and see how a more robust app can handle brainCloud authentication, as Franco suggested, you should check out our Authentication example on our Unity Examples GitHub.
Hope this helps! Please let us know if you have further questions or inquiries about this.