I have experienced this bug as well and I have found that it is a problem when Minecraft is checking valid usernames. More than likely you have migrated to a Mojang account and use your email to log in. The email address is longer than the standard Minecraft username of 16 characters, in your case it's 24 characters long.
Chris King
I have experienced this bug as well and I have found that it is a problem when Minecraft is checking valid usernames. More than likely you have migrated to a Mojang account and use your email to log in. The email address is longer than the standard Minecraft username of 16 characters, in your case it's 24 characters long.