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helper script as a workaround
Undeploying both zimlets will get you the admin console back but will adding a new user will give you the an error about the missing required posix attributes.
I posted a script that will let you add a new user from the command line:
User:Rosch - Zimbra :: Wiki
SHA1SUM: 523d990c8e8939509ee490212bca00b194f78bd3
You might find it useful.
More information is also on the Sandbox page.
Edit: I uploaded the script here since it can be changed easily in the wiki.