Endless Cyclic Restart

You can try the "Unprompted Reset" (see How to Reset My Garmin Outdoor Watch | Garmin Support Center for the instructions).

If it does not help, reach out to the Product Support - https://support.garmin.com 

Yes, "Unprompted Reset" helped. Helped for a while. But on first Brick activity change it crushed again, fortunately this time with single reset...

1. Yes, it is showing v15.77. 

2. I have loaded single Connect IQ APPS (data field https://apps.garmin.com/en-US/apps/7c83d402-4b68-4f0a-b167-7139788a19b3 ), but it is not used in Bick Multisport activities when watch crashes.

3. Recently i performed "Unprompted Reset" as trux suggested. It revived the watches, but on first sport change it crashed again (fortunately this time a single restart). 

4. I've got my Enduro2 from US Amazon, but I live in Japan. Who should I better address the US vendor or local product support if the problem persists?

Egunopor said:

I've got my Enduro2 from US Amazon, but I live in Japan. Who should I better address the US vendor or local product support if the problem persists?

You can reach out to speak with Japan Product Support to see what their policy is on the exchange but they might refer you to go through Amazon in the USA since that is the country your watch was bought in.

If you have any issues with exchanging directly through Amazon USA, please highlight my name and send me a Private Message. I will be happy to work with you directly to help you further.

Thanks for your help. Perhaps I'll wait a little bit and see how things will go on. There is nothing wrong so far with watch besides crash/reboot at sport changes. May be it is inheritable software problem and it will go away some day with new firmware updates... If not, Product Support is always welcome to fix the watch.

Egunopor said:

May be it is inheritable software problem and it will go away some day with new firmware updates

You are certainly not wrong since anything is possible. What I always research on our end when there is any serious issue reported, what are the number of reports we are receiving worldwide and when we exchanged the watch, did the behavior resolve or continue on the same software version? With the Enduro 2 series released 2+ years ago, most every software bug that might cause your issue have been addressed. At the moment, I am not seeing patterns with the v15.77 software. Suggesting, the issue is highly likely to be unique to your watch vs. a software bug causing it.

Well, I couldn't agree more - anything is possible. I simply use Multisport feature a lot and such things happened before on my previous quatix6 too and was solved after one of the firmware updates: 

https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/291645/multisport-yoga-to-open-water-change-causes-reboot

Simply on Enduro2 it happens more frequently now. Next time it goes into cyclic restarts I'll try to make video and ask Support for repair or exchange as you suggested. May be my watch is "unique" and its autopsy helps to find a reason for misbehaving... 

Since it went into endless cyclic restarts couple of times more, I have contacted Japanese support, but they do not want to deal with not-locally purchased products (並行輸入品や海外にて直接ご購入いただいた日本正規版以外の製品に関する修理やサポートは行っておりません。). Well, now I am trying Amazon US. Let's see what they say...