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 ...

I still suffer from this issue. Any insight or updates?

Thank you!

mine freezes no matter the layout

I had a similar issue as you when the previous version of octane. Once I switched my layout to the default standard startup layout c4D would close fine. Now nothing helps.

I also noticed in the bottom left hand corner of Cinema 4D it says "Octane:closeAll()" while it's frozen attempting to close

I'm running C4D Studio r18 with Octane v 4. Whenever I attempt to quit Cinema the program locks up and I'm required to force quit. Sometimes I get, "The plugin c4dOctane-R18,cdl64 has crashed" and a bug report that I've attached.

I've removed all other plugins and narrowed the issue down to octane ...