last Update WIN10 I use with Bootcamp now prevents to start with or switch over to macOS High Sierra

Problem seams resolved! I don't really know what happened. I was about to start a WIN10 session in order to control the power settings as you suggested and a message popped up saying "disk repairing ...". When this finished, WIN10 started up regularly. When I shut down and restarted the system, macOS started up. I shut down again and then pressed alt/opt at new switch on. Choice between Bootcamp and macOS displayed. Again I chose WIN10 and after start I shut down again. Then switched on with alt/opt pressed and the choice between he two systems displayed! Since one week it didn't work, now, after this "disk repairing" message, everything works properly. Thank you Loner T anyway for your support and advice. It was highly appreciated!!

Hi Lyoner T, thanks for your reply which unfortunately wasn't really helpful, since the procedure to switch from WIN10 to macOS under Bootcamp aren't workinG, as I Trier to explain in my post.

Since last friday I'm in contact with the support center, but that didn't resolve the problem either.

It seams that somehow WIN10 is now writing something in the NVRAM when shutting down. In fact when I press opt+cmd+R+P together with the on/off key and hold everything pressed for 20 sec, the NVRAM is cancelled and macOS starts again. A rather uncomfortable way to switch between the two operating systems...

So I'm still waiting for a really solving clue.

This just to update the post.

Here is the response from Diskutil:
/dev/disk0 (internal):

#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme 500.3 GB disk0
1: EFI EFI 314.6 MB disk0s1
2: Apple_APFS Container disk1 249.0 GB disk0s2
3: Microsoft Basic Data BOOTCAMP 250.5 GB disk0s3
4: Windows Recovery 482.3 MB disk0s4
/dev/disk1 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +249.0 GB disk1
Physical Store disk0s2
1: APFS Volume Macintosh HD 95.9 GB disk1s1
2: APFS Volume Preboot 22.0 MB disk1s2
3: APFS Volume Recovery 520.8 MB disk1s3
4: APFS Volume VM 1.1 GB disk1s4

So it seems that macOS indeed converted the macOS partition into APFS.

The problem is, that when i restart from a previous WIN10-Session, alt/opt method is ignored and WIN10 starts independently! Same when I press cmd+R. The only way to get a walk around is to press opt+cmd+R+P+on/off for 20 secs. After that, macOS starts, or a choice is given if I press alt/opt.

OK, tested both and both lead to the same result: after WIN10 was running and shut down, using either one or the other suggested method, macOS starts at the next switch on. But if I start again WIN10, however, the situation returns the previous.

OK, tested both and both lead to the same result: after WIN10 was running and shut down, using either one or the other suggested method, macOS starts at the next switch on. But if I start again WIN10, however, the situation returns the previous, meaning that the alt/opt key at startup doesn't work and WIN10 starts independently.

Alt/Option doesn't simply display anything. WIN10 just starts if a WIN10 session was previously started and then shut down. If I came from a macOS session, alt/opt works correctly. I tried to roll back the last Upgrades (KB4048951, 404954 and 4049011) but nothing changes. I also tried to roll back further but nothing changed either. No previous backup before problem available.