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I have this same problem in 2.0.16, using latest Apache2Triad combo.
There is mention elsewhere that going to PHP5 may cause a problem with UID variable not incrementing. I have no idea how to fix it with PHP5 and the new MySQL. Apparently something gets hosed up and the user ID variable fails to increment. Since the UID for the new user is 0, and you already have a user ID 0 (You probably as admin), you get "unable to add user".
I would surely love for someone to give us a fix! I've been looking all day for one, with no joy. And no... de-installing Apache2Triad and downgrading is NOT an option.
Thanks,
Byron