can't see the map of the track that I sent to the device and synchronised

A little more description would he helpful to understand your problem.

I’ve sent a gpx-file with a track by Garmin Connect to my Garmin Instinct for hiking and synchronised the device.

In the settings/ navigation/ type = Course

When I activate the gpx track it is recognised by the device, but when I look at the map it shows as if the settings have changed into type/ bearing.

When I activate another course that is already on the device, I can see the whole track, so the type/ course. That’s how I want to see all courses for hiking on the device for navigation during the hike.

What can I do to make sure all courses are shown with the full track on the device?

First, lets get course vs bearing sorted out.  The navigation type setting on the watch (course or bearing) only comes into play it you are navigating to a single location.  If you are navigating a course, that setting has no effect on the navigation.  See the following link for a very good writeup:

https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/instinct/246638/effect-of-the-bearing-course-navigation-setting

It sounds like you've created a course in Garmin Connect and have synchronized the course to your watch.  Then you opened a Hike activity and selected the course you synchronized to the watch. 

I don't understand what you mean by this:

peerbrok said:

When I activate the gpx track it is recognised by the device, but when I look at the map it shows as if the settings have changed into type/ bearing.

See the pics. 
I synchonised a course and the whole course is shown correct on the watch and I am able to hike with navigation on the way  

Later I synchronised another one - see pic with course in Garmin Connect - and it does not show the course on the watch, see pic. 

I'm not sure what the problem would be.  I assume that you didn't see any errors during the sync to the watch?

No problems with the sync. Just installed version 16.50 and the watch performed the update.

Might there be a relation with the Ultratrac settings? I turned that on to save battery, but now I can't find how to swith it off to see if there is a relation with the problem that the course is not shown on the watch.

Glad to hear.  Enjoy your Instinct.

Just found I have the same issue - two in fact: firstly the blank screen as shown by the OP but secondly seeing two lines, not quite aligned. This is new to me, maybe since 16.50 update, either way I really don't want to do a factory reset. Incidentally, the blank map was synced via Strava and the multiple track was via Explore.