I upgrade my OMV 3.x to 4.x and the process took quite a while but completed without any problems until I did the requested reboot.
I run headless on a ASRock Rack C2550D4I so the only way for me to see the bootup is via the Mega Rack browser app run from within Firefox due to that it's java based.
The boot is normal up until I get to the grub menu that shows the list of installed kernels. The default is a 4.14 that came in with the OMV upgrade to 4.x. If I select 4.14 to boot the console video goes to all black with a few "FAILED" prints in red along the way, so the video is really there, it's just almost all black text on a black background. It does boot all the way and I can even login on that screen but nothing shows - black on black. Otherwise, the machine is fine. It's just that the console boot screen is more or less invisible.
If I select the 4.9 bpo kernel from the menu instead, it boots just with a proper console screen like OMV 3.x did - that kernel was there already in 3.x.
Any suggestions for things to pass to the kernel in grub to get the video back to proper?