Hello - In case anyone is still having issues - I had issues with placing a video on a C4D material and on the background with Octane as well and could not get it to work.
Rendering out a JPEG image sequence instead of a .mp4 and placing it on a C4D material with "Alpha Channel" selected under the octane settings DID WORK
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using:
Cinema 4D R20
Octane 4.02.1