i think virtualmin creates 4 mailaliases by default, one of them is webmaster.
per haps this is the problem.
you can see this in the server template under "mail for domain"and another option by clicking the Edit mailaliases link in the left colom and see what aliases have been made
i have no clue really. But if you create the user postmaster, is this working?
If its not working then it might have to do with the aliases in some way and I would then file a bug.
the simplest way is now to forward the webmaster emailalias to another email address such as your private home address
and/or to ask in the bugtracker why there is a conflict (after removing the alias) and how to solve it.
Sounds like a bug…while there is a webmaster alias, by default, if it allows you to create the user without error it ought to work (or, if it can’t work, it ought to give an error when you try to create a user that overlaps with one of the default aliases). I’d suggest filing a bug in the tracker.