So the palace is not appearing in any cities wonder tab build lists.
I looked into it on the python side by making python print out every single building in the dll cache for buildable buildings; like so:
print GC.getBuildingInfo(CyCity.getBuildingListType(iGroup, i)).getType()
The result was that the palace is never included in the dll's cache of build-able buildings.
So python has to assume that it is currently under construction in one of your cities and is thus not a valid thing to build anywhere at that time.
@alberts2: you recently fixed an issue with group wonders in this very same dll cache, perhaps you could look into why the palace is not included anymore. It is a special case as it is a national wonder that you can always build 1 extra of.