This error is thrown when there is a problem with registered user.
You are trying to register the same user who is already registered.
But there is some problem with already registered user.
A simple workaround would be to delete the existing user from "Users and Groups".
To do this in Windows-
1. Press the Win+R keys to open Run, type lusrmgr.msc into Run, and click/tap on OK to open Local Users and Groups.
2. Click/tap on Users in the left pane of Local Users and Groups.
3. Delete the existing MATLABWebAppsGuest user.
Once user is removed, restart Web App Server.