Thanks for the video but I have no idea what is going on. Sorry.
The end collection error happens because of one of two reasons: because the app does not have the health permissions, or because a workout is running in another app. In your case it looks like the second reason is the cause. The moving icon for Apple's Workout app seems to indicate that there is a workout running in that app, even though there clearly is not when you go to that app.
It should be impossible for WOD to cause a workout to seem to run in Apple's app so it looks like you are experiencing a problem in watchOS which no-one else has ever reported.
Have you tried unpairing the watch from the iPhone and then re-pairing it? That often fixes strange problems which no-one else is reporting?
Sorry, I've just seen that you have already tried unpairing (I must stop replying to posts after coming back from the pub!).
It's interesting that, as well as Apple's workout icon being animated, you also don't see a small WOD icon at the top of the watch face, which is what is usually shown when it is running a workout in WOD. The watch face also doesn't show the green icon for Apple's app, despite the complication icon being animated. That is an inconsistency which, as far as I know, should not be possible and should be unrelated to WorkOutDoors.
No-one else has ever reported this and I can't replicate it, so the most likely cause is either some random bug in watchOS or something with your setup. Have you got any other workout apps installed that may be doing something strange? Maybe something that has a workout-related complication that runs all day?
Also have you tried starting a workout in Apple's Workout app and then starting one in WOD? Usually this would cause the workout in Apple's app to be ended, so it would be interesting to know if that happens for you. If WOD is unable to start a workout session in Apple's Health system then the workout in Apple's app should just keep going.
At the moment the error message tells us that WOD can't end a workout properly but this test would tell us if it can't start a workout session. If it can't then that would explain the lack of a heart rate. However it wouldn't tell us why the app can't start a workout. Or why it apparently starts a workout in Apple's app in the eyes of that app's complication, which is completely crazy!
The inability to start a workout session will cause several issues, but if heart rates are the only problem that matters for you then you could connect an external HRM to the app (via Settings / Sensors). The metrics would not be stored in Apple's Health system, but most of them should be available in WOD.