Minecraft Can't login to minecraft.net?

If I try to login ingame it just says that username or password is incorrect, it is possible to login once in a while but sometimes it just displays
java.lang.nullpointerexception if I try to connect to a server and If I close minecraft I won't be able to even connect anymore.
As I said it allows me to connect once in a while.
Added (1). So uninstalling/deleting all minecraft files doesn't help, I don't know what to do, I even uninstalled java and installed everything again.

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