Have you tried switching animals? Or changing the animals clothes, then saving, and then trying to switch? Have you tried directly getting "nekkid" then switching the base then saving even if it looks like an animal still?
I rememebr having a minor issue with getting stuck because I was trying to get the other few base achievements and it was strange..but I got out of it. I never recieved the achievements until my next log in.
My issues was I tried to directly switch into a human outfit and it got all wonky on me.