Hello,
I have similar problem and could not install more than 4 GPUS (using slots 1,3,4 and 5)
After messing up with drivers, bios, risers and everything I re-read the manual
"
The PCI_E2/ PCI_E5 slot will be unavailable when an expansion card has been
installed in the PCI_E3/ PCI1 slot."
So, as far as I understand SLOT 2 AND 3 CANNOT WORK TOGETHER!
It seems unbelievable to me (why put 5 pci-e slots on a mobo when I cannot use them all!)
I'm not quite an expert of PCI-E lanes etc., so just to make a a trial, I've ordered this expansion card:
https://www.aliexpress.com/item/NEW-Add-in-Card-PCIe-1-to-4-PCI-Express-16X-Slots-Riser-Card-PCI-E/32832542698.html?spm=a2g0s.9042311.0.0.Arny56 Let's see if I will be able to expand up to 6 working GPU's.