Fix iOS Flutter Login Bug

My Flutter-based loyalty app has been live on both stores for just over three months. Everything runs smoothly on Android, but on iOS the user-login functionality suddenly stopped working: as soon as a customer submits their credentials an error message pops up and the flow halts... (Budget: €30 - €250 EUR, Jobs: Android, App Store Optimization, Continuous Integration, Debugging, Flutter, iOS Development, iPad, iPhone, Mobile App Development)