Well just the wording of the message sounds like maybe it's a problem with your account. Like maybe it needs to be email verified or your not signed in or maybe not connected to the Internet? Lol I'm not sure but just a few ideas to check through I suppose. Can you get into other servers? (For example skyblock.org)
Well, first of all you should make sure to verify your account. That's a pretty important step... Secondly, RE-VERIFY YOUR ACCOUNT. There might have been an error, attempted hack, or a sign-in attempt from a computer farther away. Make sure your account is secure by changing your password and adding a second security question. Make sure to store the answers and the new password appropriately. If you're still having problems signing in to any server and getting the same, "Failed to Verify Username" I would contact Mojang about the problem. There might be something else that's wrong that you cannot fix alone. Hope this helps you out! Good luck!
UltraAttack do you need any further help on this? Or does all work fine now. Please try to respond within the next 48 hours, if not, I will be locking this.
