I used to get errors like this when trying to mod in forge. If you have mods check to see if you installed right and if they are compatible. If this doesn't solve it try reinstalling java with the most recent version of java.
All you have to do is click "log out" if it says "Play Offline" and log back it. This will fix it. You have to do this everytime you switch profiles. I hope this works for you. It worked for me and my bro.
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