Hi, next to the GSG part of the offer it refers to this clause -

"1. Limited time offer on annual subscriptions only. Redeemable until 12/30/2023 on new Greyscalegorilla Plus customers and renewing OTOY Black Friday customers. Must be redeemed before your scheduled auto renewal date. Limit one per ...

Been having the exact same problem and have solved it on my side by going 'edit - preferences' 'Open Preferences Folder' (in Cinema) and then deleting the Octane folder that was in the plugins folder there.
I think the plugin should only exist in 'C:\Program Files\MAXON\Cinema 4D R20\plugins'

Definitely interested!