Yesterday I updated something through the MacBook Appstore. I really do not know what I was updating exactly. I only know it was in the AppStore. It could've been an update from High Sierra to Mojave. I'm not very sure but I think it completed as the red notification badge was gone already and I quit the Appstore. I didn't do anything Special before and after that. Apps I was probably using are: Finder, Safari, Whatsapp, Todoist, Preview…
When I was finished I turned it off and the next morning I turned it on again. The Login page appears and I put in my account Password. The bar then proceeds to load but when reaching about 60/70% of the bar the MBP suddenly boots into a mode which resembles Recovery Mode. There a window reads
"The macOS Installation couldn't be completed."
In the Background there is an app/Window opened called "Installation Log/Installer Log". This is why I think the main culprit is the update from the previous day.
I have saved the logs (.txt) (only Errors, Errors and Progress, all logs) of my latest attempt to log into my account. These can be found on my dropbox. I can upload them to other Services as well if needed.
https://www.dropbox.com/sh/e70trzi7u8lb39b/AADbEd1wAj2Q02-wRfufEQhta?dl=0
Thank you for your help!
Best Answer
When you're on the crash screen showing the log, click "Utilities" at the top, then "Startup Disk", and boot from your main OS disk.
YMMV, but this fixed it for me.