Minecraft demo...

This report appears to be a duplicate of MCL-16570, which was later resolved as invalid, because this is an account issue and the bug tracker is unable to help you, as the team do not have access to the appropriate resources. Please read through this article for guidance as to why you are experiencing this problem and for potential steps you can perform, in order to fix the issue. Additionally, please see the Mojang Studios note attached to the parent ticket to see if this helps. If the problem still persists after following the given advice, please contact Mojang Support.