Optical HR area stinks

I don't know the circumstances regarding your purchase, but this doesn't sound normal at all. If you have an extended warranty, think about making contact with those people.

Otherwise consider contacting support and see what they have to say. 

[email protected]

Mine is about a month old,  there is no hint of odor, and I've worked up some really good sweats.

Yes, I'll see (or smell...) what will happen the next days and then contact support. I hope that I can do some more runs with my ankle and my leg muscles that aren't used to run anymore. The first week until I had the accident during a run the watch was fine. Okay, no sleep tracking (Beta FW fixed it) and weird altimeter readings. But no smell although I walked a bit and did some workouts for my upper body and the also injured shoulder. I cleaned the watch regularly as I did it with all watches before. Two weeks ago I noticed that smell. I can't imagine that my sweat or skin has changed. I even wash my wrist before a run.

After you clean it, is the smell gone and then comes back?

For the other issues you mentioned, the first question that would probably be asked is what software release are you on? Various releases have known various issues; and please, don't take that as my saying you should immediately go and do an update,

I like a release to be out for a bit to see what bugs are reported before I'll consider an update. If a bug creates a problem for a function I don't use, then it's Mind over Matter, if you don't mind it doesn't matter.

On Sunday I cleaned the watch with liquid soap and a soft toothbrush. For a few hours I only smelled the soap. Yesterday I rinsed the watch under warm water when I came back from my run. Today it stinks again. Only the watch, not the strap.

That thing with the sleep tracking was fixed with an update. One of the 21.xx Beta fixed it. The altimeter, no idea. I always come back below the start and I always go more floors downstairs than upstairs.

This is such an odd problem.  Based on what you said before..

"After one or two days (wear the watch at night) without a run or another sweaty workout that bad smell is back and I have to wash the watch again"

Have you tried not wearing the watch at all for a couple of days to see if it starts to stink again all on it's own? If it does start to stink without being worn, then it would seem like something has permeated the "plastic" of the watch and you are covering up the smell when you clean it.

If you can put it in a bowl of water and the watch survives then it's obviously still full sealed. If you are talking about the microscopic space between the body and the glass of the HR sensor, there's not much room there for anything to accumulate that wouldn't be cleaned with minor effort. I'll wipe off the salty sweat from the back of mine but that's about it and there is no odor.

Which leads back to the question does it start to smell bad again if not worn at all?

I'll try it with my old watch although it has battery problems after 27 months. That's why I bought this watch two months ago. After my ankle injury I want to get in shape again. It's really weird that the first weeks everything was fine. I washed the watch once or twice a week as I did it with all my other watches. As I couldn't run I didn't sweat much. When I couldn't remove the smell with water and liquid soap I took the toothbrush and scrubbed the area around the sensor. After two days, even without a run, the smell was back. Although I shower everyday, a second time after a run. Today when I came back from my run I just wiped the back with a wet cloth. Now, two hours after the shower, hmm. Still not sure if the watch was really new. Regarding the other things like several fine scratches and shiny bezel I noticed after a few days.

Yes, next week I'll contact support and see if they have another idea how to remove that smell for more than two or three days and if the watch was already registered.

I'd still go ahead with the test of not wearing the watch for a couple of days. It will be that much more information to give support and if the smell comes back when when you are not wearing it, what's left other than something to do with the watch?