I was on the mobile version of the game - and I couldn't log in for a long time (server connection issues)
I tried everything - cleaned data, uninstalled, reinstalled...
I had a good data connection - was on wifi at home - and other apps (including browser and google store) worked - so it seems to me that the problem was on the server side.
such incidents can be painful, esp when you are doing a event or a tournament..
maybe the application developers can include a debugging tool to state that the server is down instead of giving generic error messages.