The Garmin Instinct bug which caused that it was no longer able to boot. And the Master Reset.

The Garmin Instinct watch had a serious bug,
at least in firmware version 12.00 and all previous versions.
In the version 16.41 the bug is already corrected. I did not check versions in between, so I do not know exactly in which version it was corrected.

 Garmin Connect web allows naming courses using UTF-8 characters, and the Instinct watch is able to show them, BUT:
Some combinations of the name length and position of 2-byte chars
in the name were FATAL!

Instinct was not able to handle such course file, and what was much worse, after switching off was no more able to boot.

Fortunately Master Reset helps, but it is necessary to wait for circa 3 minutes.

More exactly, Instinct uses 16 bytes for the course name, while Garmin Connect sends course files
where name can be 16 characters long in UTF-8 code. Instinct probably simply cut the name to 16 bytes, but
when there was a 2-byte or 3-byte character over the 16 bytes border, problem arose.
Some examples of names which caused it:
Garmin Instinct®
Špindlerův Mlýn
Česká Třebová
Dunajská Lužná
Ростов-на-Дону
Новая Земля
Düsseldorfstraße
Kempten im Allgäu
Radzyń Chełmiński
Lyon-Saint Exupéry
Taberna La Española
Magyarszentmiklós
Кривий Ріг
Saudade do Iguaçu
Copenhagen Malmö
Αμφι-Θέατρο
בֵּית לֶחֶם
مدينة الكويت