I have seen these with with different motherboards that uses the z170 chipset. Here are the settings which helped me get pass the problem and mining with 5/6 GPUs.

In BIOS, look for and set:

1. "4G Decode" to ENABLE (this enables the PCI-E bus to use more than 4GB of memory address space)
2. FAST BOOT to DISABLE (I believe this allows the system more time to enumerate the PCI-E devices)
3. AUDIO to DISABLE (basically disable onboard audio to free up memory space)
4. Set PEG/PCI-E to GEN1 (some boards likes GEN2 so try both)
5. On some Biostar boards, need to set MAX TOLUD size to largest size and NOT Dynamic. (like 3.5GB)

Hope this helps!