I've got that same tv... Let me double check my netflix, that it's working for me, and I'll report back- at least we'll be able to rule out 1 variable (it's not working on any viera smart device/st50, which is doubtful)
? Did you need to manually activate/pair your st50 w your netflix account ever (I don't remember setting mine up)
(Unless you've already tried) try logging out and deactivating your st50 account & device pairing from netflix account control panel on nf website,
then attempt to re activate/pair/sign in to your nf account on it.
You could also try deleting netflix app from viera smarttools (or whatever the viera proprietary smart tv software is called) and redownloading it through the st50 tools/smart hub
** Unless netflix is hard-coded into the smart hub/firmware and you cannot delete the app, not sure....
--> Because it sounds as if, perhaps, your netflix app is corrupted and needs to be redownloaded OR updated.....