I found this topic here covering a similar issue The Stable (Release) Linux repositories have changed location
Selecting 40 | openHAB Related → 41 | openHAB Stable in the configuration tool is also ending in the same error message.
After this i just checked all the files in /etc/apt/ and found the following in /etc/apt/sources.list
deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
deb http://apt.pilight.org/ stable main
deb http://apt.pilight.org/ development main
deb http://apt.pilight.org/ nightly main
I commented the last 3 entries and tried the OpenHab Update again.
And its running now 
Not sure if this approach was fine but at least it seems to update to OpenHab 3.