rEFInd produces no log files, I'm afraid. That said, it's unclear if rEFInd is even launching. It sounds a bit as if the EFI is trying to launch one boot loader after another, and after each failure it's moving on to another boot loader in the boot options list, or possibly rebooting, until it succeeds.
Some questions and debugging/repair tips:
- You say that the system reboots. Are you positive of this, or might it simply be moving on to the next boot loader after a failure of the preceding one? A reboot will usually be accompanied by the re-appearance of the manufacturer's logo and POST screen.
- You say your computer eventually boots the default OS. Is this the default as set in the EFI's NVRAM (and shown by
sudo efibootmgr, EasyUEFI, and similar tools) or the default as set in rEFInd's
refind.conf configuration file? If you adjust the
refind.conf default, does that change the OS that boots?
- The output of
sudo efibootmgr -v in Ubuntu, or the boot options shown in EasyUEFI, may be helpful in diagnosing your problem.
- Depending on what
efibootmgr or EasyUEFI shows, you may be able to fix the problem by adjusting the boot order using the same tool.
- If the problem is caused by extraneous programs, like whatever Hackintosh boot loader you installed, removing it may help. Check your EFI System Partition for such programs. Clover, for instance, would normally be in
EFI/Clover on the ESP, IIRC.
- If rEFInd is malfunctioning, removing it from the ESP may be helpful; however, if you rely on rEFInd to boot Ubuntu, doing so will render Ubuntu unbootable until you resolve the problem and re-install either rEFInd or another EFI boot loader for Linux, such as Ubuntu's default of GRUB 2.
- It's conceivable that the problem is related to a damaged ESP filesystem. If so, repairing it (with
dosfsck in Ubuntu or
CHKDSK in Windows) might help. Note that damaged ESPs can be caused by a failure to disable Fast Startup and Hibernate in Windows. See here and here for information on how to disable these features.
- If rEFInd doesn't like your 4K monitor, it's conceivable that adjusting the
resolution line in
refind.conf or uncommenting the
textonly line would help; however, there's a chance that this would make matters worse, too, so proceed with caution.
- If possible, try unplugging your new 4K monitor and plugging in a lower-resolution monitor to see if that helps.
- If you can boot into your Ubuntu installation or an Ubuntu emergency disk, you could run the Boot Repair utility and select the "Create BootInfo Summary" option. (DO NOT click "Recommended Repair," at least not yet!) When asked whether to upload the report, click "Yes," and then post the URL provided here. This should provide helpful diagnostic information on your configuration, although your problem is unusual enough that clues may be scarce in the output. Still, it's worth posting that information.
I can't promise that any of these tips will resolve the problem or even produce information that will help you resolve the problem, but they're what springs immediately to mind.