I have the same problem with Reason 11 full. It's a bug of the version 5 of Studio One. I think they will correct it soon.
Meanwhile this is the roundtrip:
you can change the name of the directory where reason vst is (Reason Rack Plugin.vst3):
from:
Reason Rack Plugin.vst3
to:
Reason Rack Plugin
The ".vst3" extension on that directory is causing the problem.
Apparently after that Reason vst is recognized and works fine.
Hope this can help you.
Bye