Current status: Running.
What I did:
In order to best debug the setup process I decided to manually remove absolutely everything that had "blackmagic", "davinci" and/or "resolve" in the name (be carefull to only delete the "resolve" matches that are related to blackmagic. Ubuntu has a lot more files and demons called "*resolve*", so watch out). Every icon, every stray config, just everything.
I did nothing else other than completely cleaning my machine from old installation files. My drivers were already fine, nvidia-compute-utils-460, libnvidia-compute-460 among others were already installed, full output
~$ apt list --installed | grep 460
WARNING: apt does not have a stable CLI interface. Use with caution in scripts.
libnvidia-cfg1-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
libnvidia-common-460/bionic,bionic,now 460.39-0ubuntu0.18.04.1 all [Installiert,automatisch]
libnvidia-compute-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
libnvidia-decode-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
libnvidia-encode-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
libnvidia-extra-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
libnvidia-fbc1-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
libnvidia-gl-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
libnvidia-ifr1-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
nvidia-compute-utils-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
nvidia-dkms-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
nvidia-driver-455/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
nvidia-driver-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
nvidia-kernel-common-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
nvidia-kernel-source-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
nvidia-utils-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
xserver-xorg-video-nvidia-460/bionic,now 460.39-0ubuntu0.18.04.1 amd64 [Installiert,automatisch]
I installed the .deb that was generated by makeresolvedeb_1.4.4_multi.sh to start with a clean installation and now it worked out of the box.
So my intermediate conclusion is that the installer has a problem with other installed versions.
I have no long term testing to report but at least the app now starts out of the box.