Since hoopsnake breaks the normal left-to-right data flow in Grasshopper, it is natural that this error should occur. Yannis has already bypassed some of these "safety features," so presumably he can do this for the cluster check as well. However, you can easily get around this by disconnecting your desired cluster components from one side of hoopsnake before clustering, and then reconnect - the resulting cluster will work with hoopsnake just fine.