Did you make the BIOS settings changes indicated for your Mobo in the opening post?
Yep. Before that the mobo was not recognizing all 6 GPUs. I'll try today to install the amdgpu-pro-17.40-483984.tar.xz driver, i'll write if this fix the problem
I was thinking if this is possible to be hardware issue, as no one had problem with working 6 GPUs in Linux ....
Thanks.
If that doesn't work, consider getting the exact error and popping into the nvOC thread (https://bitcointalksearch.org/topic/mining-os-nvoc-1854250). That thread is more active and there are several knowledgeable people who help each other out, but they'll need detail.
This thread will be more active, I'm sure, after fullzero gets out the next update and rxOC becomes more useful for AMD miners.