Check your Unregistered Usergroup permissions and make sure that "This Usergroup is not a 'Banned' Group" is set to 'Yes' instead of 'No'.
^^^ Bingo. I just duplicated this issue on my v3.8.7 by changing this usergroup setting.
This Usergroup is not a 'Banned' Group (Setting this to no will cause the group to be treated as a banned group, this will prevent the sending of email notifications, show a banned error message on the permission error pages and other unspecified behaviors.)
If Guest had permission to post then I would not get the error msg. They can only read a posting but if they try to reply they get the error msg. The error msg should say "You must be signed in to reply or post to this post"