in my first attempts i had the same problem.
once everything is alright and it starts to render, the exporter somehow tells you the last
problems it had and then goes on with exporting.
be sure your directory path is clear and dont contain any weird signs or spaces.also, the last thing you should see in the path is the directory, not the desired name of your outputfile (like you could do in the internal render).
the outputfiles will be named like your project file/name. This is a bit annoying, because when you want to render different angles of the same szene,
you have to specify a new path for your output, otherwise you´ll overwrite the old renders (when they´re the same frames)
Here´s my workflow for an animation:
I set up the scene in blender and and give all the materials i want to tweak in octane a different name. I dont care about any material settings within blender.
then i hit render still image (with create/replace ocs AND write materials into ocs checked) and start to tweak the materials within octane.
When the scene is finished, i save the octane ocs. Now you just can do your camera animation, check Export Camera from scene and UNcheck both,
create/replace ocs AND write materials into ocs. hit render animation. aaaand... voilá