Goodnotes Won’t Let Me Log In on iPad 10th Gen

I'm using Goodnotes on my 10th generation iPad, and two days ago, the app logged me out on its own. When I tried to sign back in, I kept getting an error message: "Login failed, please try again."

I paid ₺899 for the one-time purchase, but what’s the point if I can’t even access the app? I contacted Goodnotes support via email, but I haven’t received any response.

This is beyond frustrating. I need a solution immediately. If the app isn’t going to work properly, Goodnotes should either fix the issue or offer refunds. Has anyone else had this problem, and how can I get it resolved? Goodnotes, please respond!