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that error because that user not exist at users table, that guest user.
but very strange if that user exist, please have a look at your database and browse the users table. please delete if any user with ID = 0.
it´s really strange. we allready tried to find the user in our dadabase, but he isn´t visable. also we tried to create a user with the ID= 0 in the database and in joomla backend. we tried to create a user and paste the same user-URL from ID=0 and to deleate him.
but everything no success.
the only thing is that the Errorpage 404 isn´t anymore here, because it´s gone with deleating the user with the URL from ID=0.
now there is opening the page with the information that the user doesn´t exsist. but the user ID= 0 is visable in the list suggested friends.