One of the things that could be going on is the filter could be plugged. I know, the tub is not that old, and I bet you will say that the filter looks fine. But skin oil is generally 'invisible' on the filter, and can and will stop just about all the water flow. You NEED water flow through the heater or it gets too warm, and triggers the hi limit system - with different results from different manufacturers of course.
So - you may simply need to use some sort of filter cleaner on your filter. Don't worry: this does NOT mean that you are going to have to super-clean your filter every three weeks. As a new owner you are just cleaning out every pore on your skin from head to toe, and it will not repeat unless you and your whole family go a few months without using the tub down the road. For years I have encouraged folks to do a water change and filter clean at about 30 days into the new ownership experience. This is a one-time thing, but it really seems to be important, and it really seems to make water care go so much easier from that point on. If the watchdog feature trips again, I would assume it's the filter.
It the watchdog feature does NOT trip again, then there's another thing that could have happened: an interruption in the power while the spa was heating. Since both are random events, you can't possibly predict when it might happen - and it's no big deal: the spa may have had the power cut for just a few moments, and the heater had some heat in it which soaks into the now-still water, and trips the warning. If this was the case, there is no harm is done. The heater cools by itself if the power interruption is a long one, or, as the restored power started everything up the restored water flow immediately cools the heater down to normal.
HTH
