@erictbar Yeah, fixed that too!
It removed you from the Premium Members group but failed to add you to the Members group for some reason, that was the error you saw. It doesn't attempt to do this until after the subscription goes through obviously, so I should make sure to figure out what is going wrong with the Member group adding and in addition add a sanity check before creating a new subscription that you don't already have an active one.
I actually built the system to be able to handle multiple subscriptions on the same account simultaneously... (like for example if, in the future, we have separate subscriptions for josei novels or R-18 novels... something like that?), but it's not really practically useful right now.