Due to human (non-bot) spam the first contribution of a user has to be reviewed and activated manually, all further contributions do not require this.
As long as a user does not have at least one reviewed and activated contribution, the user is unable to edit his profile, set a avatar, title picture or a signature.
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I, too, have observed this "vertical tear" effect,
it goes all the way up and down on the y-axis and happens always on the same x and z coordinates;
could this "connection" problem be related to chunk boundaries?