I imported a material from the Octane DB/Live DB that rendered first the first time, but then rendered black in the next render.
I've tried reimporting the material etc and it seems that after the render has finished the assets/textures (that was saved in AppData\Local\OctaneRender\cache\texCache\assets) of the material get deleted (including the assets folder).
The image might linger some if it's opened in Blender's Image Editor, but if you try to change its settings etc it will appear as missing ("Can't load...").
I've solved it by resaving the images to another location (packing them might've worked as well), but that's a tedious workaround..
I made a screen recording of the issue, although it didn't capture the Octane DB window it shows the problem in Blender.