I've unfortunately tried all of the above, to no avail. I will continue to trouble-shoot and get back if there are any solutions.
Edit:
I went back and tried again, with your suggestion of putting in some time intervals for breaks. It seems to have done the trick. I assumed that 'randomize break times' would take over, and allow me to leave those values empty.
I also tried deactivating breaks, which didn't work, weirdly.
Anyway, to anybody with the same issue - ensure you have numbers in the break times and you should be okay.