same problem here.
all i did though was just installing a new skin based on the light skin.
i did not do any chnages to the skin itself. so its basicaly the light skin. so this worked for a couple days without problems.
now i cannot log into the admin anymore alltough client login works fine.
i get a white screen with the input fields and some basic text like login, username:, etc.
when i try to login i get:
Code:
Log In
Administration Console
Your Session Has Expired
Loading ...
Username:
Password:
New password
Confirm
Log In
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Zimbra Administration
JavaScript error encountered in method ZaZimbraAdmin.prototype.startup
The requested resource (/public/admin.jsp) is not available
when i hit reload i get
Code:
The requested resource (/public/admin.jsp) is not available.
when i restart the whole box i usually can login as an admin.
any idea?
how can i uninstall that skin properly again?
i removed the skin directory and restarted the server
zmprov gcf zimbraInstalledSkin still listens the skin
the following does not do the trick 
zmprov mcf -zimbraInstalledSkin <skinname>
running open source edition on debian sarge stable in vm ware.