When the anonymous permission is set for a document or folder you can access it directly without any authentication process u2013 as an anonymous user.
Later you log on with your user, so, when you try to access the CM iviews (that you changed the property "Authentication Scheme" to anonymous u2013 according to Note 837898) it will ignore the authentication and the system u201Csupposesu201D that the last user (in this case anonymous) is trying to access that doc. Although your user has access to that file, anonymous user doesnu2019t have.
I donu2019t have any log and I donu2019t know what kind of authentication scheme you are using to access the portal. According to your description Iu2019m only guessing what is happening (http traces could help you to confirm if the system still considers the anon user).