/etc/apt/sources.list.d/buster-backports.list
That is the other file. Comment the line by putting a "#" in front.
Yes, I tried the omv-release-upgrade. No success.
That is why the guide put the steps 5.1 and 5.2
If it is not working to run the command again, restore your backup, fix your sources until 5.1 and 5.2 are successful and run the omv-releaste-upgrad