I’ve got a workaround…
Tested with Windows 10 playing in Borderless Window Mode:
Step 1: Move the coursor over your Mapmarker - press and hold rightclick
Step 2: press and hold leftclick while still holding rightclick
Step 3: Release rightclick while still holding leftclick
You should now be able to modify your Mapmarker - however I had the problem that I couldn’t move the mouse now - mousemovement was just moving the map instead of the coursor.
To solve this issue:
Step 4: Press the windowskey on your keyboard
( windows startmenue should pop up overlapping your gamewindow )
Step 5: Hover your mouse over the delete button ( / ) and select it
Step 6: click delete
Hope that helps anyone…