Instinct 2 Solar Tactical just boots into the service menu PROBLEM

You may have already tried, but not shown in your video:

1. Hold upper left CTRL button longer than shown in the video - approximately 30 seconds - until watch shuts off completely.  Then restart.

2. Have you tried simply pressing the BACK button while the service menu is displayed?

HTH

1. Yes i've done that. Just did it again for 30 sec. and after restart it boots straight back to the service menu.

2. Pressing the BACK button while the service menu is displayed, just lets you "scroll" through the different options in this menu

OK, I'll give it one more shot:

Have you tried holding the BACK button while the service menu is displayed?

HTH

As you can sadly see, holding the BACK button just jumps it to the last section in this menu. Pressing it again and you're just back at the start. 

I guess my last two options are 1. - waiting for an answer from the Garmin-Support or 2. - waiting until the battery is fully empty and hopefully it will start normally again after charging it. Somewhere i read that this solved the problem from one or two guys. 

2721958 said:

after restart it boots straight back to the service menu.

In that case it almost certainly means that the DOWN button is stuck in the pressed position, since holding the Down button while starting the watch brings the Diagnostics menu. You will have to clean and unstuck the button, and then the watch will boot normally. Try letting the watch for a longer time in a container with clean lukewarm water. You could use a lightly soaped water, but only shortly, since it could damage the sealings. You can also try brushing it with a soft fine toothbrush.

I thought about that also and already put the watch in wstrr for 10 mins. Just the way Garmin says on their website. But I'll give it another try with lukewarm water and a longer time.