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.
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.