I was able to find some instructions or recommendation for dual boot configurations in this case OpenELEC/Windows 8.1* from a post on a different website.
The recommendation that may help you would be:
A- Make sure to disable "Secure Boot" at BIOS level (Boot tab).
B- Boot Manager issues:
If none of the systems starts, this probably means that the secure boot option was not disabled.
If one of them starts (this may be the issue in your system), that means there is a problem with the boot management. To fix this it’s necessary to go back to Ubuntu, with the USB drive again and go to "Boot Repair"
More instructions on how to access "Boot Repair" can be found in the source post below.
If any additional questions are present, do not hesitate to advise me.