macOS startup not possible after Bootcamp Windows installation (APFS/Startup Manager/Recovery Mode)

Hello!

System: MacBook Pro 2016 with Touch Bar, macOS 10.14.6

The backstory (actual problem see below):

The logic board failed on my MacBook, had it replaced by an Apple Service Provider. They transferred the Macintosh HD contents from the old SSD to the new one, Bootcamp partition was not transferred.

macOS started up fine and looked like before, however Bootcamp Manager didn't allow partitioning because the System was on macOS Extended journaled file system and had to be converted to APFS.

I therefore launched Recovery Mode and converted the Macintosh HD volume to APFS using Disk Utility. The process ended with some error message I don't remember.

After this, I was able to restart macOS and launch Bootcamp Manager to do the Windows installation. This worked fine and I'm now able to use Windows normally.

The problem:

I hold the option key when booting to launch the Startup Manager, but it now only shows the Windows partition, not macOS.

I launched recovery mode and found this:

(German, please tell me what might need to be translated)

I'm especially confused by "AppleAPFSMedia" which now seems to contain my Macintosh HD volume and another, greyed out volume, called "VM".

I tried first aid on the Macintosh HD volume and got this message:

It says:

Checking saving system

Exit-Code for saving system check is 8.

Repairing saving system.

Exit-Code for saving system check is 8.

Checking or repairing the saving system failed.

Action failed.

When trying to choose Macintosh HD as my boot volume, I get this error message:

It says:

You can't choose this volume as your boot volume.

Creation of Boot-Caches on the Boot-Helper-Partition failed.

What do I do now? Is Macintosh HD corrupted? How can I launch macOS again?

I'd be very grateful for any advise on this, thanks in advance! :)