I had exactly thesame problem a few days ago when I installed my 4th GPU.
I changed "gen4" to "gen2" in the bios, aswell as "enable 4G encoding".
Not sure what bios you have, so just Google these terms for your bios version, it might help.
Good luck.
Exactly this. Ive got a Gigabyte motherboard and tried to install a 6th card but Windows wouldnt boot (just a cursor flashing)
After a quick google I enabled 4G in bios and went straight in.