If your Minecraft launcher is not cracked, it seems as if the best way to fix it is as follows:
"You may need to refresh your login session. To do this, click "Switch Users" in the launcher, and logout. After this, log back in, and try the game again."
Although you've stated that you've tried this, maybe give it one more shot and follow these instructions directly.
I'm not really sure what else could be causing the issue.
Good luck!