Though I have never installed Ubuntu on a Mac nor have I ever used rEFInd, but this problem may be causing this issue. I expect so because this could be a driver problem. Was rEFInd installed by default with Ubuntu on a Mac?
As given here
nomodeset
The newest kernels have moved the video mode setting into
the kernel. So all the programming of the hardware specific clock
rates and registers on the video card happen in the kernel rather than
in the X driver when the X server starts.. This makes it possible to
have high resolution nice looking splash (boot) screens and flicker
free transitions from boot splash to login screen. Unfortunately, on
some cards this doesnt work properly and you end up with a black
screen. Adding the nomodeset parameter instructs the kernel to not
load video drivers and use BIOS modes instead until X is loaded.
X server has better drivers than the kernel counter part. With nomodeset
, the kernel does not loads the video drivers during the boot-process and it uses the modes provided by BIOS. The video drivers are loaded when the X-server starts. The drivers in X-server could be able to handle the hardware and show graphics properly.
You may see this forum for setting nomodeset
in rEFInd, or this answer for setting it in grub