The fix was already enabled, but an old version could have caused it. I work with old scenes from time to time. Sometimes even from 2016.It could come from a scene saved from some particular versions of max plugin, when the 'Material layer' was introduced.
Could you check and post the version that you see in Tools tab, rollout '1.9 version support' : in 'Current Scene', it should show the 'Octane version'.
I'll try to create a similar scene..
Also, could you try to enable 'Fix invalid material layer group' in Preferences, Material tab ? That could fix the issue..
Thanks
Will keep an eye on it and report back if I get this error again.