Hi Flo,
Maybe it's too late for an answer but maybe it will be usefull for other people.
I had exactly the same problem I think.
The error message from RebusFarm Quickcheck was : Error Pluginshader corona shared (1040749) are not supported
I didn't have this problem before Corona 6 (c4d).
So after hours of debugging my scene in all the way possible, I found what's going wrong and its easy to solve : some materials from Maxtree Collection (and in your case, maybe from other model collection or selfmade materials) contains Shared Properties, you just have to rebuild your material by replacing original texture in place of all textures named with "texturename(shared)" or "texturename(bitmap)".
To be more accurate, in my case the problem seems to only exist with MAXTREE PlantModel vol016 (I have many others collection and at this point, I do not have Error message with it).
Maybe it's not the solution you expected but in case of rush, you can do this to be able to render with rebus....
Best regards.
CrazyBum2