I can't play APEX

I can't open APEX. The login screen keeps loading in a loop and I can't play. I tried clearing the cache and repairing it, but it didn't fix anything. This is the second time it's happened; the first time was about a week ago. I tried various things and eventually, uninstalling completely and reinstalling fixed it. But a few days later, I can't play again, so I want to know the cause and how to fix it.