Hi, I have same issue with both Reaper 64bit and Cubase LE 6 so don't think the issue is with Reaper itself. Tried numerous things such as disabling AV and firewalls, given full write access to VST folder and disabled routers firewall in case it is just the registration part that is being blocked. PC is couple of years old, Win 10 64 bit (fully up to date), Core I3, 8GB and using Zoom R8 as audio interface.
Any update appreciated otherwise if I get any progress will let you know.