Update resulted in "Only official released binaries are allowed to be flashed(Recovery)" error

/e/ version:
Device model(s): SM-G960F/DS
Device rooted: yes/no

Summary

When I was updating my SM-G960F two weeks ago to the most recent version of e/OS, my phone turned black and displayed the error message "Only official released binaries are allowed to be flashed(Recovery)". I bought the Samsung Galaxy 9 with e/OS preinstalled from e.foundation. I am new to this, so I am a bit overwhelmed.

What I tried:

flashing the most recent e/OS using download mode, but I got the same error.
Easy Installer does not work either.
booting into TWRP, TWRP is not installed.
USB debugging is turned off
OEM is locked
Smart Switch fails to detect the phone (at least for the Emergency Recovery function)

I figured now that the reason why I cannot flash an OS is because the KG state is set to prenormal. I read that one can bypass the KG state by downgrading to Oreo official stock firmware and then I should again be able to flash e/OS.

Is this how I should proceed or is there a better way? Is there any way to backup my data

The problem

Steps to reproduce

I don't remember the exact version of the update, but it was the most recent version around the 30 of March.

Reinstalling e/OS fails because KG state is set to prenormal (I assume at least).

What is the current behavior?

When I try to start the phone I get the message "An error has occurred while updating the device software. Use the Emergency recovery function in the Smart Switch PC software."

What is the expected correct behavior?

For now bypassing KG state so that I can flash e/OS again.

Technical informations

Relevant screenshots

Solutions

Workaround

Possible fixes

Bypass KG state and flash custom OS

Checklist

License
Translations
Documentation
Test plan