My iPhone has an problem logging in on a specific app

I have an app for work. Whenever I try to log in on my iPhone, it gives an error: "Warning. Check your internet connection". It doesn't matter if I use mobile data or WiFi. If I try to log in on the iPhone of my parents, it works with both phones. I did a reset to factory settings and I had set it up as a new iPhone. When I downloaded the app, i could actually log in. I had made a back up of my iPhone before so after the new user test I put the back-up back on the iPhone. I try to login on the app and well, it didn't work anymore. So can anyone help me to login? There is clearly something wrong with my iPhone/back-up.

iPhone 7 plus

iOS 11.2.6 (15D100)