Hi.
I have the same issue. Normal 600+ calorie work out is now 126. I have lost my Garmin HR and am using a compatible one. All data on that front looks good. The link you have given is no longer working. Is there a fix?
What type of workout? running? If so, obviously 24KCal is wrong.
One of the easiest and most accurate ways to estimate the Kcal burnt when running is "distance(km) x weight(Kg)", and it is very little depending on pace or HR zone. (it is, but very little compared to the total figure)