This resolved the issue for me on both servers I experienced it with.
Code:
zmprov mcf zimbraInstalledSkin bare zimbraInstalledSkin froggy zimbraInstalledSkin harvest zimbraInstalledSkin lavender zimbraInstalledSkin rose zimbraInstalledSkin sand zimbraInstalledSkin sky zimbraInstalledSkin steel zimbraInstalledSkin vanilla
Looks like the upgrade NULLified "zimbraInstalledSkin" for some reason.